You lose the bet. zip is there and I also used it for the upgrades. The php log is full is deprecated comments and errors about inbound email but nothing about diagnostic processes. the problem is likely permissions and the way SuiteCRM mangles file directory permissions without my permission. In that respect SuiteCRM is unique in the CRM world.
diagnostics does work on the softaculous 8.6.2 demo using admin login which tells me it must be a problem related to my installation.
I know how tricky permissions can be, but after a stable configuration they really should be stable. I see many people who constantly set and reset their permissions, but I avoid that at all costs. It’s a matter of getting a few settings right, and then not messing permissions up by logging in and changing files with other users.