1. I'm a long time Reaper user on my late 2012 Mac mini. I always stuck with Mountain Lion ("if it ain't broke...") until recently as my browsers became unsupported and I decided to update my OS.
2. I installed High Sierra on a separate partition to test it and when I was happy decided to update my Mountain Lion partition - which contained all my Reaper projects and plugins etc.
3. BUT, when I tried to do this I found that High Sierra had been withdrawn and Mojave was being offered instead - Hmm
4. Stupidly I updated my Mountain Lion partition to Mojave and deleted my High Sierra partition - at least I had backups of everything.
5. Belatedly I discovered that my Audio Interface people (Tascam) only had drivers up to High Sierra and that my US-1800 (now a legacy product) was in a queue for Mojave driver development.
6. I won't bore you with the convoluted path but I finally managed to recover the Mac's original Mountain Lion OS which I was able to update to High Sierra. Naturally I restored my backed up files, including my Reaper project files (not including any added plugins)
7. I got the High Sierra drivers for my audio interface and downloaded a fresh Reaper64 which runs fine - ie when simply adding a sample wave file.
8. The trouble started when I tried to open a recent project:
a) The standard 32bit warning was ok; as I read on here so I ignored that,
b) I then got "Registry Error" warnings which I'm not sure about,
c) Then it would tell me about all the plugins that were missing. I still had my Mojave partition, which contained all my old folders, from when it was the original Mountain Lion, which included loads of the Reaper plugins that I was now missing. I copied as much of what looked relevant to similar folders on my new High Sierra, naively thinking this would satisfy my existing Reaper projects which had used these plugins in the past.
d) Now Reaper would simply crash during loading any project containing any non-native plugins,
e) Finally, I found that, at this point, it would still run fine by starting a new project BUT, if I tried to add any of the plugins I had "imported" from my old partition/OS, although "add FX to track" would happily LIST all these plugins, when I tried to LOAD them it reported that "the following effect plugin could NOT be loaded"
HELPPPPP!