Firewell thats odd, i can only randomly troubleshoot from here...
- turn off steam cloud so none of the changes might be overwritten by badly saved files.
- make a backup folder on your desktop (or anywhere outside of the steam folders), move your "\cache ; \cfg ; \download ; \downloadlists ; \maps ; \replay ; \screenshots ; \workshop" folders from \tf\ to that backup folder (those are all the folders where i think files are being changed usually) and then verify game cache again (to redownload now missing files).
- start tf2 using the -autoconfig launch option (and ideally remove all other launch options).
- verify that steam actually looks for tf files in that directory (rightclick on tf2 in your steam library > properties > local files > browse local files; this should open the "C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2" dir).
if all this doesnt help i really have no idea anymore, there might be something wrong with your windows or hdd pointing at non existing directories that tf2 interprets as non valid. thats just a rough guess tho, theres probably an easier solution to it that i just dont know...
The autoconfig thing helped, the game finally works. Thank you so much!