Apparently it's a font rendering issue in Windows, the Unicode character for diameter simply does not exist, so it's nothing NAT can solve on its own. I also use the Scandinavian vowel Ø (ALT+157 or ALT+0216) or its lowercase variant ø (ALT+155 or ALT+0248), which were the only available characters in extended ASCII before we had Unicode. Funny thing is that the empty set Unicode symbol ∅ (ALT+2205) works just fine, just not the specific diameter symbol U+2300, which allows machine reading to uniquely identify diameters, not confused with writing in Swedish or something. I would think that the diameter symbol is used infinitely more frequently than the empty set symbol.

But I am no machine and do not "need" this character, Ø or ∅ do just fine for my purpose, I was just wondering why it broke NAT. It would be better to handle this kind of error more gracefully rather than losing the whole post. I noticed attachments also break down when there is a standard accentuated letter in the file name. I don't expect a fix either, as NAT support is all volunteer work, so let this thread just provide some explanations for these small bugs.