Can you dump the full error stack from the logs? It's hard to see exactly where the error is actually occurring.
There are a couple of notes that may apply:
2032559 - CredentialStoreManager lookup method must adhere to public API
2012183 - reverting BLS SDK method changes introduced in 14.0 SP5
Take the time to search as well, I only spent a few min and there may be more.
If you haven't already I recommend taking the time to update the custom action with the latest SDK and implementation requirements.
Regards,
Christian