I solved this problem with a complete reinstallation of Labview 2013 SP1 (repair did not work).
I came across a possible reason for the error: I imported Glyphs from the ICON EDITOR after the first installation and added my own glyphs. These files are usually stored in the LabviewData folder inside "My documents".
I assume that there has been some issue, when reading the glyphs from there. So maybe it would not have been necessary to reinstall, but to only delete the Glyph folder, start Labview and Icon Editor and reimport the glyphs.
Forget my last answer.
I do not know what I did wrong, but I have the same errors again... This makes me really angry...!
I'd like to recommend you to please post this question into the Discussion Forums rather than in the community, this because the discussion forums is a more active community were you will receive more and faster replies from different users with a lot of experience.
LabVIEW Discussion Forums
Thanks for the suggestion - in this case I was going for a more targeted audience than a general forum posting and in the end I think the issue for me was more that my IT dept were doing something behind the scenes.
That day, they automatically uninstalled my TortoiseSVN, upgraded firefox, applied several Windows updates and probably a whole bunch of other stuff I didnt notice. The next day, after shutting down (and even more updates processing) and restarting everything returned to normal. I'm not sure if any were directly related but as so much happened and then the problem dissapeared, it seemed the most likely caulprit to me.