diff --git a/packages/next/src/client/components/router-reducer/reducers/server-action-reducer.ts b/packages/next/src/client/components/router-reducer/reducers/server-action-reducer.ts
index 2d580a4a0f..4626db0dec 100644
--- a/packages/next/src/client/components/router-reducer/reducers/server-action-reducer.ts
+++ b/packages/next/src/client/components/router-reducer/reducers/server-action-reducer.ts
@@ -106,10 +106,9 @@ async function fetchServerAction(
       )
     : undefined
 
-  let isFlightResponse =
-    res.headers.get('content-type') === RSC_CONTENT_TYPE_HEADER
+  const contentType = res.headers.get('content-type')
 
-  if (isFlightResponse) {
+  if (contentType === RSC_CONTENT_TYPE_HEADER) {
     const response: ActionFlightResponse = await createFromFetch(
       Promise.resolve(res),
       {
@@ -136,6 +135,19 @@ async function fetchServerAction(
       revalidatedParts,
     }
   }
+
+  // Handle invalid server action responses
+  if (res.status >= 400) {
+    // The server can respond with a text/plain error message, but we'll fallback to something generic
+    // if there isn't one.
+    const error =
+      contentType === 'text/plain'
+        ? await res.text()
+        : 'An unexpected response was received from the server.'
+
+    throw new Error(error)
+  }
+
   return {
     redirectLocation,
     revalidatedParts,
@@ -167,9 +179,7 @@ export function serverActionReducer(
       ? state.nextUrl
       : null
 
-  mutable.inFlightServerAction = fetchServerAction(state, nextUrl, action)
-
-  return mutable.inFlightServerAction.then(
+  return fetchServerAction(state, nextUrl, action).then(
     async ({
       actionResult,
       actionFlightData: flightData,
@@ -207,9 +217,6 @@ export function serverActionReducer(
         )
       }
 
-      // Remove cache.data as it has been resolved at this point.
-      mutable.inFlightServerAction = null
-
       if (redirectLocation) {
         const newHref = createHrefFromUrl(redirectLocation, false)
         mutable.canonicalUrl = newHref
diff --git a/packages/next/src/server/app-render/action-handler.ts b/packages/next/src/server/app-render/action-handler.ts
index 72cf877ed1..fbca7f88c9 100644
--- a/packages/next/src/server/app-render/action-handler.ts
+++ b/packages/next/src/server/app-render/action-handler.ts
@@ -251,7 +251,7 @@ function getAppRelativeRedirectUrl(
   host: Host,
   redirectUrl: string
 ): URL | null {
-  if (redirectUrl.startsWith('/')) {
+  if (redirectUrl.startsWith('/') || redirectUrl.startsWith('./')) {
     // Make sure we are appending the basePath to relative URLS
     return new URL(`${basePath}${redirectUrl}`, 'http://n')
   }
diff --git a/test/e2e/app-dir/actions/app-action.test.ts b/test/e2e/app-dir/actions/app-action.test.ts
index fec271dcbf..bc11adb5a4 100644
--- a/test/e2e/app-dir/actions/app-action.test.ts
+++ b/test/e2e/app-dir/actions/app-action.test.ts
@@ -7,7 +7,7 @@ import {
   waitFor,
   getRedboxSource,
 } from 'next-test-utils'
-import type { Request, Response, Route } from 'playwright'
+import type { Page, Request, Response, Route } from 'playwright'
 import fs from 'fs-extra'
 import { join } from 'path'
 
@@ -91,6 +91,56 @@ describe('app-dir action handling', () => {
     ).toBe(true)
   })
 
+  it('should propagate errors from a `text/plain` response to an error boundary', async () => {
+    const customErrorText = 'Custom error!'
+    const browser = await next.browser('/error-handling', {
+      beforePageLoad(page: Page) {
+        page.route('**/error-handling', async (route: Route) => {
+          const requestHeaders = await route.request().allHeaders()
+          if (requestHeaders['next-action']) {
+            await route.fulfill({
+              status: 500,
+              contentType: 'text/plain',
+              body: customErrorText,
+            })
+          } else {
+            await route.continue()
+          }
+        })
+      },
+    })
+
+    await browser.elementById('submit-transition').click()
+    const error = await browser.waitForElementByCss('#error-text')
+    expect(await error.text()).toBe(customErrorText)
+  })
+
+  it('should trigger an error boundary for action responses with an invalid content-type', async () => {
+    const customErrorText = 'Custom error!'
+    const browser = await next.browser('/error-handling', {
+      beforePageLoad(page: Page) {
+        page.route('**/error-handling', async (route: Route) => {
+          const requestHeaders = await route.request().allHeaders()
+          if (requestHeaders['next-action']) {
+            await route.fulfill({
+              status: 500,
+              contentType: 'application/json',
+              body: JSON.stringify({ error: customErrorText }),
+            })
+          } else {
+            await route.continue()
+          }
+        })
+      },
+    })
+
+    await browser.elementById('submit-transition').click()
+    const error = await browser.waitForElementByCss('#error-text')
+    expect(await error.text()).toBe(
+      'An unexpected response was received from the server.'
+    )
+  })
+
   it('should support headers and cookies', async () => {
     const browser = await next.browser('/header')
 
diff --git a/test/e2e/app-dir/actions/app/error-handling/error.js b/test/e2e/app-dir/actions/app/error-handling/error.js
new file mode 100644
index 0000000000..436e4a1e3c
--- /dev/null
+++ b/test/e2e/app-dir/actions/app/error-handling/error.js
@@ -0,0 +1,17 @@
+'use client' // Error components must be Client Components
+
+export default function Error({ error, reset }) {
+  return (
+    <div>
+      <h2 id="error-text">{error.message}</h2>
+      <button
+        onClick={
+          // Attempt to recover by trying to re-render the segment
+          () => reset()
+        }
+      >
+        Try again
+      </button>
+    </div>
+  )
+}
diff --git a/test/e2e/app-dir/actions/app/error-handling/page.js b/test/e2e/app-dir/actions/app/error-handling/page.js
index 56cd1db6fa..2c93a95f0b 100644
--- a/test/e2e/app-dir/actions/app/error-handling/page.js
+++ b/test/e2e/app-dir/actions/app/error-handling/page.js
@@ -1,8 +1,11 @@
 'use client'
 
+import { useTransition } from 'react'
 import { action } from './actions'
 
 export default function Page() {
+  const [, startTransition] = useTransition()
+
   return (
     <main>
       <p>
@@ -18,6 +21,15 @@ export default function Page() {
       >
         Submit
       </button>
+
+      <button
+        id="submit-transition"
+        onClick={async () => {
+          startTransition(() => action())
+        }}
+      >
+        Action that triggers an error
+      </button>
     </main>
   )
 }