Segementation faults are mostly caused by hardware problems. You shoud check the hardware of the host server with memcheck. Also make sure that the memory limits of your vm are not too low.
The segfualts have nothing to do with ispconfig itself as ispconfig is written in php and you can not cause a segfault with php code. Segfualts can only be caused by bugs in the php version itself or the surroundung os enviroment like hardwrae problems.
Where can the LOGLEVEL_DEBUG in /usr/local/ispconfig/server/server.php be changed?
In the ISPConfig configuration file: /usr/local/ispconfig/server/lib/config.inc.php