Hi,
I repeat: could you give us an URL of your forum?
OP needs to check all the forum's activated plugins. Otherwise, it could be impossible to determine where the "0" string comes from in this circumstance.
I have not yet solved.
But I just discovered that the "0" is a hyperlink! and clicking it opens up as follows:
Disabled the Thanks! plugin
Thanks system (2.6.3)
by Whiteneo
and the 0 disappears!