Perhaps, in some version(s) of the engine, game_load does not always return the correct value on failure?
(It seems the return value is not received / evaluated in function continu().)
Of course, it seems 'you' have already indicated that nothing was changed in the script, level, etc. between save and load.

irrelevant but...
file_open_read probably wasn't designed for that exact purpose.
Seemingly, if such a function is not already provided for that specific purpose, a file_exists function for that exact purpose, might be useful. Of course, if file_open_read is effective already, then...

Maybe also irrelevant but...
What does the function body look like for gevecht() ?

Is the function continu() called in an inappropriate fashion (maybe more than once in a certain period, etc.)?

Quoting Secret_V

...a crash in the function which invokes game_load...