Not sure if it might perhaps be somehow related to the bug (in 10.15.4) mentioned in this thread:
https://forum.cockos.com/showthread.php?t=234181
But does the project open with FX offline?
- By using the Open with FX offline option at the bottom of the 'File > Open Project' dialog
- Or by holding CMD + SHIFT (which is the global shortcut) while opening the project
Just to rule out that the issue is perhaps related to a specific plugin used in the project (which might have compatibility issues with Catalina).
Otherwise, did you already try installing/using the dedicated 'OS X 10.15 Catalina' Reaper version?:
http://reaper.fm/download.php