My drive is loaded and reached its limit. Tixati keeps complaining about something called "core2.dat.temp". Could anyone please elaborate what this file is used for and why Tixati tries to change it (in volume, mb's as well)?

the .dat files are the tixati inner-workings files.
the core2.dat file saves your core configuration settings.
it sounds like you have run out of room in the folder you have your .dat files.

Yes that is right, hence why I asked the question what this file actually entails. My question still remains, why is it that without me changing anything in the config, the size of the file increases?

you must be adding something to the program, new torrents, rss feeds ...