We have the same problem with MODE SOLUTION (Version 2021), it reports
"LumApiError: 'appOpen error: \n Failed to start messaging, check licenses...'"
We previously used 2019b and 2020a, and did not have this issue. It only came up when we upgraded to 2021.
The MODE Solution window actually opens properly. It is just that it does not return the proper handle back to Python, and thus we can not continue from the Python script.