Subscriber looses access to content unexpectedly

Hi,

I've mysterious case so any advice is appreciated. There is a site which sells Premium Content access using Commerce sp and Commerce sp paypal combination. Users are able to obtain auto-renewed access via recurring payments. Most subscribers are happy but there are some (not much, 6-7 reported cases) who unexpectedly looses access to content ("Premium Subscription" role) and / or aren't able to access content after payment. There is no errors in Watchdog, Paypal IPN messages looks normal, all "related" (mean possible related with issue) patches are being installed: system came to me pre-configured with all patches related to validity check and other suspicious cases. I've tried to analyse subscribers data and found that in all cases "Subscription validity" field is empty and that is all useful information so far. Any piece of advice will help, since I'm ran out of ideas and can't even imagine a place from where debugging could be started.
Yet another strange thing is that .info file doesn't contain version number so I don't know actually what version is used, guess most stable one (7.1.2).

Thanks!