... there is always some offset. example the input is 0.05mv but output 0.38V instead of being 0.01. Is that normal? (Gain is 205 (RG=1K), and ref is tied to ground).
You are right, this Proteus model seems has an offset error.
With shortened input pins (Uin=0) and w/o (opened) R1 (G=5) the
output is 302 mV instead of the expected max. 1.25 mV (250 uV x 5).
Try out the offset compensation via the ref pin (datasheet, fig. 2)
or ... do not use it in Proteus (sorry :-). There are a lot of good OPAs ...