Undefined context error when user resets password

30 minutes later, that's me again, and again it doesn't work (same error)
So, I rebuilt rights, and now it works.
For how long?

Thanks for this prompt answer.
This morning it works again, without changing any thing in the settings. But I am not confident an I will test again periodically
I will do my best and help you and find the culprit event, but I am not a professional coder, only a part time hobbyist.

Another thing I noticed:
When Acces Policy module is running,
and when user opens the "edit profile" form
he doesn't see the “visibility_users_groups” field.
When the admin opens the "user profile" form he can see it.

This is good, because only the admin can set the user groups,
but I dont know if it's the normal behaviour.
And may be this is the reason of the issue?

Xdebug? I am not at all familiar with this tool. Seems it needs a learning curve. Sorry I will not be able to help in this way.
But in the meantime I found a way to trap the error more precisely, at least on my production site.
I explain how to:

Initial status:
A user, named “testuser”, tries to refresh his password. It works!
Repeat again a couple of times. Still works. So far so good.

Disconnect “testuser”
Connect as “admin”
Go to “admin/config/performances” and clear the cache.
(Notice: cache lifetime is set to 15 minutes on my site, may be not the best value?)
Disconnect “admin”

Now “testuser” tries again to refresh his password,
and BINGO! Here is the error in line 256.

Disconnect “testuser”
Try again to refresh password. It works!

Hope it can help.

See attached file
Thanks in advance for your help

Only for memoty!
Something strange:
Access is controled as expected in my main site with this config, but...
There is probably someting wrong in it because if I import it in a clone of the main site, and then delete cache and rebuilt rights, access control is not working at all.
An hour later:
if I import this config in a clone of the main site it works, but ONLY for new content.
I have rebuilt rights (admin/report/admin dashboard/rebuilt rights) but It doesn't work for existing content . I have to edit and resave save existing content to make Access Policy work
15 minutes later
I just remember the solution in this thread. !!

My site is running a custom subtheme of Bartik.
This subtheme has now been running for 2 years without any issue.
I can reproduce the error very easily

I just made some tests with native Bartik and also with Olivero, and...
In these themes I cannot reproduce the error.

So, the error could come from
- the fact I am using a subtheme
- or from my custom code
I will investigate and keep you informed.

result of investigations:
While running Bartik: I cannot reproduce the error

While running a subtheme of Bartik: I can easily reproduce the ERROR "line 256"

I have removed nearly everything from my subtheme so that it is now as simple as possible with only 2 files:
bartiksub.info.yml

name: BartikSub
type: theme
base theme: bartik
description: 'Theme enfant pour site G-FAMILY.'
version: '1.0'
core_version_requirement:  ^9 || ^10

bartiksub.theme

<?php
// that's all

So, my conclusion is that the error is coming from the fact that I am using a subtheme.
What do you think?

NOTICE: This post was written before your sent the pach
You are on the right way!
This is what I did:
in access_policy.module (line 327

/**
 * Implements hook_entity_field_access().
 */
function access_policy_entity_field_access($operation, FieldDefinitionInterface $field_definition, AccountInterface $account, FieldItemListInterface $items = NULL) {
/*********************************
if ($items && $items->getEntity() instanceof AccountInterface) {
    return \Drupal::service('class_resolver')
      ->getInstanceFromDefinition(EntityOperations::class)
      ->userFieldAccess($operation, $field_definition, $account, $items);
  }
*******************************/
  return \Drupal::service('class_resolver')
    ->getInstanceFromDefinition(EntityOperations::class)
    ->entityFieldAccess($operation, $field_definition, $account, $items);
}

This is the result:
bartik & bartiksub ("empty")
No more error. :-)
But of course the field is now displayed in user form, and this is not good ( I could hide it in css, bad practice )

Attach:
Crop of user fields
Crop of user fields form

The patch seems to work in my test site. :-))
I will test in production site and keep you informed.

I appreciate your prompt and efficient assistance. Thank you so much.

Are you by chance restricting taxonomy terms with an Access Policy? In particular, Groupes Utilisateurs?

No, Acces Policy only uses "Groupes Utilisateurs" taxonomy terms for content acces control, but doesn't control acces to terms.

That's me again.
May be I was wrong in my privious post.
I dont intend to restrict access to "Groupes Utilisateurs" taxonomy terms, but may be I did it by mistake, or insufficient knowledge of Access Policy settings (there is a learning curve for me because translation in french is only partial and I still need to understand many things)

See attached file.
Field access.png

So, I just made a test after selecting "Libre" (free) instead of "Droits" (Rights).
It doesn't change the behaviour. Only the patch does a good job!
I tested it in production site and it also works.

I can confirm-
- There is no "taxonomy" access rule in my settings
- Only one "content" access rule