MacOS 10.15.7, I just installed RM 18.104.22.168. I tried to add a mother with the surname “Unknown”, and there was already a person in the DB with the same first name and surname. I selected “Continue adding mother” and got an access violation. I restarted RM and tried again. This time there were two candidates, so I selected the one I had just added (no birthplace, unlike the first one), and got the access violation again. I restarted, found the newer candidate, and changed the surname as a workaround.
I’m not able to reproduce this issue on the Mac. I am on Sonoma, but I wouldn’t think being on Catalina would be the issue. Try running your database tools under File, Tools.