Same for me: win xp sp3, avast 10.3.2225 (italian version) and thunderbird 38.2.0.
Disabling avast protection error disappear.
Moreover avast stopped PDF24 (freeware virtual printer) to function, the error is related to writing of a temporary file (same as TB). Seems to me that AVAST 10.2.2225 has some sort of filter for temporary files.