This was also asked in the newsgroup. My answer there was:
You are writing to a fixed filename in each of the parallel threads. Sometimes when you try to open the file, another thread will already have the file open. In such a case the fopen will fail and return a negative number. You do not test for that possibility, and you use the negative number in fprintf. fprintf recognizes that the negative number cannot be a valid file identifier and tries to make sense of it as a format instead.
To this I would add that 'filename_lab.txt' is taken literally. Try
to get a lab-dependent name.