[TYPO3] BE users (not) belonging to groups

[Tomas Mrozek]

CASE:
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I have a site with a couple of BE users and two BE groups.
When I look at the users via root -> edit user, they are properly
assigned the group that they should belong to.

PROBLEM:
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When I look at the users via Tools -> Users and compare their settings
they seem not to belong to the groups (even the admin users). And when I
login under the user's account, the user truly doesn't inherit any
rights from the assigned group.

QUESTION:
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Has anyone come accross such a problem?
What might possibly be wrong?
(my head just doesn't get it)

TYPO3:
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v 3.8.1

INFO:
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I created the site 6 months ago and everything was working fine. During
that period some people with administrator rights might have done
whatever the adminstrators might, event though they claim they haven't
done any significant changes.


Thank you in advance for any hint!

Tomas Mrozek