Users get flooded with messages about connection problems

Question

We have an installation with TYPO3 8.7.45

since half a year the editors get flash messages with the text:

Connection Problem

Sorry, but an error occurred while connecting to the server. Please check your network connection.

That is independent from current activity, also it seems to be triggered more often when they are editing CEs.

with my admin-account I have no problems, if I change user and su to an editor account I also get the messages:
just open a CE and wait. about 1 minute later the users get flooded with about 40 Flash messages.

In the TYPO3 log there are no entries for the editor or my account for the moment the messages appear.
In the apache log no error occurs.

In the moment of the messages a JSon call comes back but with http-code 200.
it went to https://<mydomain.tld>/typo3/index.php?ajaxID=/ajax/usersettings/process&ajaxToken=5fbc9e1b13fae602644589e9ec63ffadb72b35d9

in the returned JSON nothing special about errors.
It looks like a user record from a system with a lot of pages, so there are lot of page-tree states and fileadmin-tree states:
(I needed to shorten it (marked [...]) because questions aren't allowed to have so much characters.)

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[...]
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How can I get more Information what connection has the problems?

Answer 1

maybe a serverside firewall is responsible for that. and blocks the suspischouls looking traffic