hello, these type of problems are related to session handling and issues that might had been fixed in the latest joomla and VM releases.
to make sure your sessions work OK:
- make sure you are using only one domain and not all 4, since on some hostings the session is not shared:
http://www.domain.com vs
http://domain.comvs
https://www.domain.comvs
https://domain.com- make sure to use latest joomla + vm as there were problems with sessions already reported
- watch your #__session table for the maximum session length and any misbehariour upon writes/reads, etc... large sessions caused by large cart may take some time to get stored and if there are too many ajax calls on the site, those session may run into incositency (proper locking should be used by DB + VM + Joomla ... )
best regards, stan