1.) The initial preset of patches in 2008 may today be a total different one, because better and better patches have been developed?
The original TI|1 was released 2005, with the Ti|2 refresh in 2009, ten years ago now.
There have been a few OS updates since the date you specified (2008) - notably TI OS 3, 4 and 5, which added stomp-box Overdrive/Distortion effects, Tape Delay, additional Character models, new Filter types (Vowel, Comb, multimode Vari-Slope and X-fade algorithms), Modulation/Step-Sequencer, and two additional EGs.
So there may be more 'modern' patches since then that are more varied and make use of these newer features, but most patches will make use of the classic features that came before these updates. (Even the Virus B that was released in 1999 sounds great, and it has none of the features of the later Virus C or TI series!)
The last full feature update (TI OS 5) was released December 2012, though, seven years ago now. Since then the synth has only received bug-fix updates, no additional features.
2.) From where you get/find your patches today?
Same as Oli, I tend to program all my patches. I do remember using the Vengeance incubation soundsets (the early free ones), They were on the original Access' website for free download.
I've got several hardware subtractive synths and when I was starting out I deconstructed the sounds that I liked to see how they were made. Doing so gives you a great idea of how audio programming works, and these tactics can then be transferred and used on any other subtractive synth.
3.) Are you mostly buying patches or do you find 'open source' patches somewhere?
I used the free downloads part of Access' site. There are loads.
4.) Is there anywhere a library or inventary where e.g. the 20 best patches in each category (pads, arps, leads, bass, ...) or by genre (like 70s, analog, 80s, ...) are listed?
'Best patches' are subjective, I imagine everyone has different favourites, thus you can't really list them. I do remember Access populating a 'best of' bank, which became the factory presets, but this was in the Virus-C days, long before the TI.
5.) Do you know of a library which contains best non-techno patches like piano, church, organ, strings, violine, trumpet, percussion, ... patches?
Sorry I don't, but then again I never looked for those types of patches for the Virus, I tend to use ROMpler synths or physical modelling for natural acoustic/orchestral sounds.
Wonder why there no sub-forum arround virus ti patchs in this forum, where such discussions are done?
You're already posting in the Sound Design section.
Maybe I misconstrue... Do you mean a download section? Copyright is an issue, I can't simply duplicate all the downloads from Access' site, or host copyrighted patches from other sites, but people are still free to post links to their own patches.