To see the configuration that fop2 uses run
/usr/local/fop2/autoconfig-buttons-freepbx.sh
Look at the "extension" and "context" parameters in every extension definition, when you transfer a call, the call will be redirected to that extension@context. Suppose that you have extension 100, the config in a default freepbx setup would be
[SIP/100]
type=extension
extension=100
context=from-internal
If you have something like *100 in extension instead, then the button is misconfigured.
Most probably you have an issue with your configuration/dialplan, but I cannot tell you where/how or why. Transfers from fop2 should work the same as transfer from the handset.