Unable to select membership plan

Suddenly, and without having changed any site settings, I am unable to assign a lifetime membership plan to a user. When I select the plan and save the user record, It appears to save properly, but the plan reverts to the lifetime free plan. I used to be able to do this in the administrator area, but something is clearly causing it to no longer work. Any ideas?

Do you have lifetime plans configured to create subscriptions? That's required if you want the subscription to remain otherwise it's rather superficial and just meant to satisfy exclusivity conditions. You can enable this in CBSubs > Settings > Global by setting "Create Subscriptions also for free lifetime plans" to "Yes".

Yes, I've always had that setting set to Yes.

Strange, do you have any auto actions in CB Auto Actions configured to act on CBSubs that might be removing the subscription? If not try enabling debug mode and maximum error reporting in Joomla global configuration as maybe something is interrupting the storing process and the subscription isn't being stored.