I believe this was done in the last plarium update, this was a forced update stopping you playing the game unless you updated.
The old system worked like this:- each plarium opened gave you 10 game accounts and you could choose what account you wanted to open and play but only one account at a time, using sandibox you could open another plarium giving you options to another 10 accounts. 4 or 5 lots opened would give you 40 or 50 accounts.
with the latest plarium update 10.9.0-0.0.0 when you open plarium in sandbox you can see all ten accounts but only access the first one, if you try to choose switch to another of the 10 say the 2nd account, the list you choose from goes blank and the game seems to freeze giving you no access.
I think plarium are trying to make people only have 10 accounts not 50 and I do not think it is a problem with sandbox. I also find that opening 5 cases of plarium play and running 5 instances of the game maxes my cpu out. I have a good cpu 32 gig of ram on a 3xs gaming laptop.
may I suggest once you have 5 open and start to get problems open task manager and check you cpu and memory usage. you will be running 5 lots of flash wrap and this eats up your cpu.