Grayed-out area when trying to choose venue in frontend

Hi
when I try to add a new event in the frontend and hit the "choose location" button, it opens the window to choose the location, but it blocks the website. I am not able to choose any location or return to the add event page. (s. picture)
This is in Mozilla, in Chrome there is just an empty windows opening, like there is no connection to the database.
We use JEM 4.3.4 on joomla 5.4.0 with PHP 8.3.27
New events in back end have no problems.

It happens with different logins.
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Hi, it seems, you have not the right to use these venues.
What are your settings? You are in an admin group, what are the settings there?
The wanted venues are yours? Are they published?
​​​​​​btw: new posters can't upload images (only after 
10 posts).if yo want, you can send them with PM.

I used two
one is the superuser account I also use für the backend access and that I used for the creation of the venues.
the second one has publisher rights.
I will look into the user rights again tomorrow, but have the impression, it is not linked to the user setups.

Hard to know what the reason is, could be anything. This needs investigation.

Turn on the Joomla error loging and see if you have any interessting messages.

If it is possible, make a backup of the files and database and reinstall JEM right over the existing installation (I guess you don't use any core hacks, they would be replaced). Just take the same JEM version.
Let's see if this takes us any further.