After about a week of bug hunting I found that a corrupted PipelineCache.esopc and/or ShaderCache.cooked lead to a crash at login before the character screen is displayed. Before that I had about a month of random crashed — especially in PvP and in the middle of a battle. I don't know if that is related but just in case I mention it.

Suggestion: If you experience crashes at login or in game it might be an idea to delete those files. As then names suggest they are cache files and are recreated when the game is started the next time

@zenimax: Applications should not crash when reading cache files. Errors should be caught and the cache file should be deleted and recreated automatically. Who reimburses me for a week of lost gameplay?