Hi

I noticed this problem after upgrading to 4.9.1, and I decided to wait as it was reported as an issue. Im not sure if I am repeating somebody, as I havent read all the posts here, as I get "DotNetNuke Error: - Version 04.09.00" errors randomly when browsing this site.

Anyway, after I upgraded the whole site got very slow and the sql server worked constantly at full load. I was reading about similar problems and caching issues, and attempted to replace the file for the cache provider from 4.9.0, that is assembly version 3.2.2.23754 DotNetNuke.Caching.FileBasedCachingProvider.dll. 

The users online module now works and the database is at normal workload. How clever it is to do this I dont know...

This is work-around works for me.  That is, backing up the version 3.2.2.23891 of DotNetNuke.Caching.FileBasedCachingProvider.dll provided in DNN v4.9.1 and placing version 3.2.2.23754 provided in 4.9.0 in its place resolved the updating of UsersOnline table.  I found no difference between Scheduler modes (Timer or Request) and left it on Request, which was default on my install.  Not sure if that was a setting brought over or not, but I digress.

Nice work Mamen.

Would be good to get some insight from a core member on possible ill-effects besides what we can see is a positive work-around here.

Thanks,

Adam