It seems to me that Univention removed particularly the package version of OpenProject for UCS 5.0 from the app catalogue. It was there, otherwise I wouldn’t have been able to upgrade my systems to UCS5 at all.
For some reason the compatibility of the current OpenProject version in the app center has then been downgraded back to UCS 4.4 only - with the result that existing installations can´t be managed anymore within the management console of UCS5. At least as it appears to me - the app itself is running smoothely in its container with above mentioned workaround for the proxy.
I haven’t seen any communication on this on OpenProject or Univention forums, so I can’t say if it’s a temporary issue or if support for OpenProject in UCS is going to be discontinued.