I had the same issue.
In the end I resolved the issue by deleting all files in the cache folder.
WARNING - worked for me, might not work for you. BACKUP profile files first!
Running on Ubuntu Linux, all GoldenCheetah files are in /home/username/.goldencheetah/athletename folder
In that folder is a folder called ..cache
I backed up the whole profile to another location first, the only deleted files inside the cache folder. Likely the file rideDB.json was the only one you might need to delete, but can't validate, but suspect this was messed up.
After deleting, first load of goldencheetah is slow, but all my PMC charts came back. Prior to that all non PMC charts displayed fine, but PMC charts were all flatlined.