This has happened to me during various flights in Australia (and not anywhere else):
Requesting a step climb, say from an inital cruise altitude of FL360 to FL380, it is denied by ATC saying that it is below minimal altitude. Instead, i'm instructed to climb to FL600 (!!!). The only way to stop hearing this demand for hours is to quit P2ATC, restart it and file a new flight plan with the requested FL.
Normally, I just enter the new FL in the Cruise Alt field, re-file and request a climb to.
P3DV4.5 , current Navigraph data