I'm having problems installing .sis files on my N93. Around 50% of them don't work (they give a "installation not supported") error message, even though the software in question is specifically said to be compatible with the N93. For example, yesterday I tried to install a python script (which is basically just a text file) contained in a .sis file, and I got that error.

In the app manager, I've set installation to "on" (I think it's called "all" in the English version though. I have the Swedish version). Online cert validaiton is turned off. The phone is not locked down by the operator in any way, because I didn't buy it through an operator.

Has anybody been able to solve this problem?

This is *NOT* a problem.
What you are trying to do is install applications that are incompatible with Symbian OS 9.1 S60 V3. You need compatible applications. sisx files are the default for installing in S60 V3. sis files are the old ones used for Symbian 8.1 S60 V2 and older
If the s/w is supposed to be compatilbe with S60 V3 then make sure you are using the correct installation file since most software has different files for different versions.