You can append ?safe_mode=no_themes%2Cno_plugins to your url to activate safe mode as an alternative:
https://service.slaa.network/login?safe_mode=no_themes%2Cno_plugins
Though looking in the browser console, it does look like the error is coming from a progress bar component of the procourse installer plugin as Pfaffman suggests.
What errors did you get on the rebuild when you tried removing it?