Once the problem starts there seems to be at least three different commands that are not working any more:

1) mtl_set -> w1301 when it tries to read a fx file from the shaders folder (PATH is set correctly)
2) file_open_read -> returns 0 when it tries to read a file from the user's roaming folder
3) file_save -> engine crash when it tries to save a file into the user's roaming folder

All those commands are working fine until the point where suddenly all those commands are not working anymore.
Until now the problem occured only when the game was running for a long time. Last time it occured the tester played about 90 minutes until the issues started.

We use a nexus of 200mb and there were always at least 50mb left free.
From the task manager the game uses about 800mb of memory and VMMap shows a heap of about 1.1GB to 1.2GB.
Unfortunately we don't have memory values at the timepoint when the problem occurs yet.

Edit:

By the way ... level_load seems to work fine although the other commands fail ... our tester was able to walk through several rooms after the w1301 occured.
After every level_load we have a file_ope_read that returned 0 after the w1301 and the engine crashed when the tester tried to save (file_save).
Don't know why level_load is working while the other commands don't.