After upgrading from 7.4 u90/u91 to a newer version, some Content Pages throw errors and cannot be edited

Issue

After upgrading from 7.4 u90/u91 to a newer version, some Content Pages throw pop-up errors: "Your request failed to complete".
If you try to edit those pages, an empty page is shown.
In the browser console some errors appear: "Uncaught (in promise) Error: true"

Environment

Upgrades from 7.4 u90/u91 to a newer version

Resolution

First of all, follow this article to print some useful logs: "Your request failed to complete" error is displayed on the user interface but there is no more information about it.
After that, if this message is shown in the log file after that, you are likely to be affected by this issue:
```
[SessionErrors:66] Adding key fragmentEntryContentInvalid
```
Enable traces (set level to ALL) for com.liferay.fragment.internal.renderer.FragmentRendererControllerImpl and check the log.

You may see something like this:

com.liferay.fragment.exception.FragmentEntryContentException: FreeMarker syntax is invalid.__
The following has evaluated to null or missing:_==> featureFlags [in template "template_id" at line 2, column 37]__----_
Tip: If the failing expression is known to legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use [#if myOptionalVar??]when-present[#else]when-missing[/#if]. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??_----__----_FTL stack trace ("~" means nesting-related):_ - 
Failed at: #if featureFlags["LPS-182728"]?? && f... [in template "template_id" at line 2, column 32]_---- [Sanitized]

If so, check your Form Fragments. You may have duplicated some of the out-of-the-box Form Fragments in versions 7.4 u90/u91. They had some freemarker code that checked a Feature Flag. That code must be removed.
- After updating your fragment, remember to propagate your changes.

Additional Information

Creating Form Fragments
LPS-194363 removed the code from the out-of-the-box fragments