Hello Volker,

this symptom could be related to the "restart engines after each game" option (a tournament setting you may enable using Arena) and, in addition, CPU demanding emulated devices starting simultaneously. As a potential result, the Almeria is reset (your screen-copy) while Arena has lost communication with the emulated device. Of course, the tournament is stuck.

As a basic test, try manually loading several/many MessChess engines in Arena and check if long waits happen, high CPU usage, and finally try accessing the configuration per engine (UCI options, such as the computer level). If the configuration window is empty, Arena has lost control/communication with the MessChess engine, and would not be able to send or receive any move.

If so, try the trick I shared time ago here. Then, re-try loading several MessChess engines, check if it is better (and please let me know the result )

Hope this helps,
Eric