Random unit test error PHP class does not exist

I’m just new to Travis CI and also Glpi plugin devel. I tried to find similar issue for some days now, and couldn’t. Hope this is not a stupid question.

I’ve adjusted Travis CI configuration files and got my commits to a Glpi plugin (php) working all right, but the Glpi branch 9.2/bugfixes usually yelds RANDOM errors stating a “Tested class xxx does not exist for test class xxx”.

The problem is: there is no error ever using other branches (namely 9.3/bugfixes, 9.4/bugfixes, master of Glpi source), using PHP versions 5.6, 7, 7.1, 7.2 and nightly.

The bigger problem is: the error are random. I’m testing 4 static methods of only one class. Sometimes there is error in only one random method, all other three run ok, sometimes the error appears only for another method, sometimes are two methods with error and two without. And, if I re-run the build, the error disappears or change to other random method. That is making me crazy, as there is not a single point to find the problem.

If someone has encountered something similar, I would like some advice. Maybe something with the environment, lack of some resource. As it comes and goes without changing anything, I’m really without options here. Thank you.