Jira Issue links are set to /undefined/

[frhaa]

Hey, we recently updated our Jira Core Version from 8.20 to 9.0. We are running 3 different projects on this instance which share most of the data fields, worklflows, behaviours and so on....but for some reason one of the projects always changes the URL from https://company:8443/browse/KEY-976 to https://company:8443/undefined/browse/KEY-976 on Refresh/F5 and everything breaks. The data is loading correctly but all the buttons and functions are not working because the /undefined/ because its also integrated in the JavaScript functions (e.g. Edit Button). This only effects one project, the other projects work just fine. We also updated all plugins (Scriptrunner, Rich Filters, etc), did re-indexes of all projects and use the same server.xml like before. Which Jira function is setting the /undefined/ into the URL or at least at which part of building the HTML page?

Edit: It also routes us to https://company:8443/browse/undefined/browse/KEY-976 when clicking on issues from the filter/search view