Howard Miller

I think he's saying (sorry if I'm picking you up wrongly, Petr) that if your code hasn't been through HQ's checks and balances then you're not trusted. That means nearly all optional plugins. I don't find this surprising... not all developers are motivated to take that sort of time. It's even worse if those plugins then include other libraries from unverified sources.

I am not a security expert.

Personally... I'd strongly recommend not doing that. Grading Assignments moves through various states and it's much better to create a new activity for each Assignment. I'm not saying that what you're attempting isn't possible, it just goes against how (as far as I can tell) Assignment was designed to be used.

Sorry - I have no idea what you mean. Especially "12.50 for 8 grades = 100 ". Can you explain in more detail / example / screenshot of your Gradebook settings?

Seen this before... https://moodle.org/mod/forum/discuss.php?d=454555

So which version are you running in production now? Do you still have the problem. If it can be avoided, don't upgrade a site with a mysterious problem - the risk of making it even worse is ever present. Expecially (as has been pointed out) with 5.1 thrown into the mix.