'ERROR: Cannot acknowledge event' raised with Acknowledge, but not raised with Acknowledge and return.

An acknowledgement has an allowed length of 255 characters.

If it exceeds 255 characters, the acknowledge button wil raise an error returning:

ERROR: 'Cannot acknowledge event'
Value: "acknowledge input text" is too long for field "message" - 456 characters. Allowed length is 255 characters.

If you exceed 255 characters and hit the 'Acknowledge and return' button it won't raise an error and won't save the text either.