After an upgrade to build 9346 the customer was not able to send proofs and deliveries when tracking was enabled.
Deliveries were failing with the error:
Sending worked with the OOTB tracking click formula.
The custom tracking clicks formula acxMultiBrand that was no longer in use somehow got activated during the upgrade. As the customer is using the Web Analytics Connector, this formula should not have been used.
To resolve this issue the value of acxMultiBrand option was updated to the value of the default tracking formula (that has been used in the first place by the customer).