If it happens "suddenly", it's because another product has used the key which formerly used by ParkMe that, with GSX installed, has been replaced by a message stating ParkMe is not available anymore.
Not that this changes anything with or without GSX installed: if there was no GSX, you still had that potentially conflicting key (because ParkMe was installed and active), the only difference is the message would have been different, saying that "ParkMe is not available while flying".
The key was CTRL+Shift+P, so it's likely something tied to a 3rd party airplane system, perhaps related to Pressurization or barometric Pressure.
You can:
- Change the key used by ParkMe with the "Change key assignment" menu under the main "Couatl powered product" menu
OR
- Change the key assigned to the same key as ParkMe in the other addon
OR
- Remove the fsdreamteam\Couatl\ParkMe folder