Exec error op_readKN

Hello everybody. I’m new at using freefem and I started using a code which may have to work of a simulation of a mechanical problem.
Running the code I repeatedly got this exec error:
Fatal error in op_readKN(array)
Can someone help me in undestanding the reason of this? I didn’t really found what this kind of warning stands for.
Thank you in advance!

Please provide a minimal working example (MWE) if you want us to help you out. This is probably due to a mismatch in array sizes, between the one defined in your script and the one you are reading from file.