Thanks for your reply.
We are not using GetResponseEx() to receive unsolicited messages. We are using an event handler (as per the manual) to receive unsolicited messages. We are using GetResponseEx() to continuously query the PMAC for other information (such as P and Q variables).
GetResponseEx() works fine and the event handler for unsolicited messages works fine independently.
The issue occurs when an unsolicited message comes in at the same time a reply to one of the GetResponseEx() queries comes in.
I64 is set to 1 as per the manual but this does not resolve the issue.
Note that when the issue occurs, the unsolicited reply also does not show up in pewin32pro, indicating a bug at the pcommserver level.
Any advice is highly appreciated.