It is not an avera to take someone to a din Torah and accuse them of stealing from you. You can make a direct private accusation to the person you suspect.
The case discussed in this thread in not theft.
This is a different case. To succeed in Jewish law it also requires a level of proof that is not used by the secular courts.
That is why the accusation is an avera – because it brings a Jewish person to non-Jewish law and thereby creates a hillul Hashem. It is against Jewish law to take a Jewish person to a secular court.
That is why I wrote above –
(by prejudging)
and