Exception: Serialization of 'Closure' is not allowed in serialize() using CiviCRM with Omnipay & Mollie
The answer appears to be that something is getting put in the session by the omnipay code that the CiviCRM caching code doesn't like. Here's the issue that describes the full details: 

https://github.com/eileenmcnaughton/nz.co.fuzion.omnipaymultiprocessor/issues/17

And here's a patch Eileen includes that seems to have solved the issue:

https://github.com/eileenmcnaughton/nz.co.fuzion.omnipaymultiprocessor/commit/933806960e1a4c5968e9e6145feffb79eb77b415

FWIW - most of the thread is involved in getting good debug info.