OpenProject error after Univention upgrade

I’m not able to access to OpenProject after upgrading Univention to 4.3-0 errata3 release version. When I try to open that app, the following error is displayed:

Proxy Error
The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /openproject/.

Reason: Error reading from remote server

Apache/2.4.25 (Univention) Server at Port 443

I also tried both to restart the univention server and to unistall and reinstall OpenProject, but the error has not been solved.

Can anybody help me?