multiphase stops "The fl process could not be started."
 

Hello, I am a beginner and have a problem with multiphase simulation. I tried to simulate bubbles in a vertical pipe using euler-euler. the simulation actually runs very well, but it sometimes stops at random times.
The error code is:

================================================== =================================
= BAD TERMINATION OF ONE OF YOUR APPLICATION PROCESSES
= RANK 0 PID xxxx RUNNING AT ---
================================================== =================================
The fl process could not be started.

When I restart the simulation at the last saved step, it continues normally until it stops again at another step.
I do not use UDF or similar. The process runs on double precision but only one core.
Does anyone have any idea where the problem could be and why the simulation is aborted?