PayPal setup goes 404

Event Tickets 5.29.3.1
WP 7.1
Theme Didi Lite or Twenty Twenty-Five

Hi, i ran into two issues today.

A ticket order with PayPal could not finish. The process on PayPals site went through (PayPals test booking of -/+1$ arrived at the bank), but PayPal was not able to return the client to our order confirmation page. No explicit error, just the circle icon circling forever (no booking of the actual ticket price).

Clicking the reconnect(?) link in ET>Settings>Payments>PayPal did not show any effect.

So I decided to delete and setup the PayPal Connector again. Clicking on the “Connect Automatically with PayPal” results in an 404 error:
“Not Found – The requested URL /wp-admin/&displayMode=minibrowser&partnerDomain=https://domain.tld was not found on this server.”

All caches deactivated and cleared.
Webserver’s error logs show the 404, but nothing useful otherwise.

The setup is years old and has worked successfully in the past.

Any help is appreciated, thanks in advance!

Cheers,
Jørgen