Thank you !!
Pcmurray, mikev1, you are right, the P.rag curling is a real issue, but I have problems also with very flat "plastic" papers over 250 g/m2...
However, widening the plateng gap before loading paper is a good idea, but I have a doubt:
if I set the "widest" setting from printer menu, this setting will be overriden by the "standard" setting saved in my printer-driver ? I don't like the print quality with wider platen settings .
In the meantime I've tried a workaround loading the A4 sheets in the same way of sheets greater than A4... It really works and the paper is always loaded without misalignments. Yes, this method is slower than the normal method.