This occurs when trying to play a PBEM turn. I have a JP CV that needs to be placed and when I click "ok" to start to select the placement hex this happens. I tried loading the turn twice and it happened both times.
EDIT: I tried it a 3rd time and instead of clicking "ok" to place the reinforcement after viewing replay I clicked "cancel" and then started moving and later came back to place the unit and it worked this way without error.
Thanks for the report and this is a very odd one as I'm unable to repeat it at all on my end. It has been reported by others with repeatable steps, but just does not repeat on other systems for some reason.
Not sure why at all as usually anything repeatable is universally repeatable, but good to see you were able to figure out the workaround.
In the meantime I'll keep trying to understand why this occurs as it does and ideally provide a fix.