Anyone had experience about the mast base(or mast joint) getting stuck in the board after session. It s so tight that I can't twist it a bit. Any reasons for that and how to twist it off it without damaging the board ?Thanks for help.

Stupid question but have you try removing the vent screw?

Yes sir

Well, the reason I can see is maybe some sand on the screw or in the square bolt but I won't be able to help or give any advice unfortunately...

rubber mallet really light taps. Yes they will stick in there as the brass t nut corrodes bad because it is contact with stainless bolt, in old boards sometimes the alloy mast tracks would also corrode & cause the stick.