Page setup lost, no CSS formatting, inactive buttons

I installed piwik on a shared server at my provider to monitor small websites.
Everything worked fine for some years.
I do not regularly check the statitics.

When I now sign in to my piwik instance, I get a completely unformatted browser interface.
I can see the content unformatted in plain text form.

In the text I can see the following message:
Oops… während der Anfrage ist ein Problem aufgetreten. Möglicherweise war der Server temporär überlastet, oder eventuell haben Sie einen Bericht mit zu vielen Daten angefordert. Bitte noch einmal versuchen.

The problem is permanent.
In addition, the many buttons on the page do no longer work.
I can see the button animation when hovering across the button, but there is no action when pressing the button.

On my wordpress homepages I can see that piwik tracking is still active.
SO the database seems to be ok.

I manually updated to the current piwik release as described, but see no change, the piwik page is still unformatted with inactive buttons.

Anything I can do to correct the presentation of data?