Caching of blocks from the contact_form_blocks module leads to "Validation error"

The contact_form_blocks module allows block caching for its provided blocks.

When the user now logs-in in parallel into the system via two browsers with different sessions the cache can get corrupted and a "Validation error" is detected by FAPI in drupal_validate_form() (the session id is used as part of the fom token validation test; see drupal_valid_token() ).

As solution contact_form_blocks can provide an admin setting whether to cache the blocks.