I had the same error happen with a user. I can't upgrade to any plan with this user now. The page just renders blank after selecting from the membership upgrade form.
It seems this happened after I had opened this particular user from the CB User Manager and changed the status of a previously subscribed plan to delete. After trying to upgrade this users account to another plan from the front end upgrade form, it just rendered a blank page. I tried other plans and it does the same. From the CB User Manager I also tried to activate different plans. When I open it after saving the new plan it shows that it didn't save at all or activate the plan.
The history log shows this in a few recent entries with this user:
Message: loadSomething::subid 454 with planid 2 could not load.
Table affected: Plans
Table Key: 2
Is there a value I can change in the database to get this user back to a normal status or am I stuck having them make a new account? Help is appreciated. Thanks.
I noticed this issue was able to clear itself after a few hours. Strange thing but it started working for me with out doing anything just waited it out.