No, it doesn't seem to be related to the SpamFilter.
It rather looks like invalid input, as it's the following statement which fails:
unicode(value.value, 'utf-8')
with the (truncated) value.value being:
value.value == 'Previewbekannte Inkompatibilit\xc3\xa4ten) lassen sich...i R\xc3\xbcckfragen lange Recherchen n\xc3\xb6tig werden.\r\n\r\n'
The fragment of value.value displayed in the backtrace is valid utf-8, however the error message says that the error occurs at position 2816, i.e. somewhere in the truncated part.
It's a bit hard to say what's going on without additional details.
Can you monitor the traffic and get a full dump of the HTTP request?
