This error can appear when you attempt to sync a Group in Event Temple with Mews. The error originates from Availability Blocks that were created in Mews and then synced to Event Temple using the External Identifier method. The reason for the error is due to the Availability Block in Mews not having a Release Date.

To resolve this error and have your Group sync successfully, you will need to set a Release Date for the block in Mews. At this time Event Temple does not support Groups with daily Release Dates or no Release Dates.