After upgrade gitlab-branch source plugin to version 645.v62a_b_6fce8659 Secret Token changes from plain text on jenkins configuration to credentials selection made gitlab webhook stop working:

Old version

Recent version

After update webhook stops working 

In jenkins logs a erros occurs:

Tried to create a new credential with tthe same secret text configured in gitlab but does not work.

If roll back and set the secret text again in jenkins configuration, webhook works again