I had this problem before, it happens when I try to save with the name generated on my system "/home/yonatyt/Documentos/simutrans/pak64.german/demo".
(https://github.com/Yona-TYT/imgs/raw/main/Captura%20de%20pantalla%20-2022-04-02%2011-40-25.png)
should be fixed in r10710