Invalid type "Subach HL-7" found in loadout of mission file...skipping
As a pretty wild guess, sounds like what happens when you try to run the FS2 campaign with FSPort as a mod, which is pretty frequent among newbies who don't know the retail campaign is selected by default, and hence that they have to switch campaign before they can play.
Trying to put into 127 byte buffer:
fighter2v-01-debris-shine.dds
Sounds like what would happen if someone installed FSO in Program Files or something silly like that : absolute filepath getting too long.
Can hardly formulate any other hypothesis on those two without a proper debug log. I also suggest you tell them to come ask here directly, because a proper, solid and reliable FS community is much better to ask support than a random unknown German forum.
EDIT:
He used the installer.
BAD. The installer is broken and has been for ages.
STOP DOING THAT.
MANUAL INSTALL guys. Srsly.