Thank you for your reply.
The SOCKET_READ instruction block returns a USINT array with the message received (data parameter), as well as a UINT (RxAddr parameter) with the size of this message. I have to set up a loop to read the message content character by character, and to do that I have to access each individual item in the array. Unfortunately the array indexes are DINT, therefore the variable used to address each array position needs to be DINT. In order to compare the indexer variable to the upper bound of the array I have to compare a DINT with a UINT, which apparently is not possible.
Unfortunately, the MOV instruction block is also picky about data types, so it didn't work
Error 1 myDINTvar: Expecting a UINT/WORD type variable