Hmmmmm..... interesting experiences both of you are having here.
First, let's define the things that can cause your board to round up.
#1...... the rail to rail (roll axis) trim of the board is upwind rail lower than downwind rail.
Why..... because your stance is putting your weight/center or gravity upwind of the boards centerline, or you are putting weight on your front foot which is upwind of the centerline, without compensating by putting your back foot "heel on centerline" to compensate for the added weight upwind of centerline and bring the board back to level (rail to rail).
#2....... Raking the rig back too soon.
When you are starting to get underway (either a short board or a longboard here) you need to ensure that the rig is fully perpendicular (the axis or line drawn from center of the mast to the center of the boom end) to the fore and aft axis of the board.
If you do not get the rig perpendicular, but rather start to sheet it in before you really got the board and rig "aligned" in the "T" position, the board will instantly try to turn upwind. It's doing exactly what you are telling it to do if you rake the rig back as you sheet in the sail to power it up.
#3...... Mast base is positioned too far back on the board (but this is easily compensated for by keeping your rig a bit more forward (tipped forward over the universal joint to put more of the drive from the sail further forward when getting underway.
For sure, Marek, you need more time on the water to discover how much more sensitive a narrow board is than you F-Type 148.
The F-Type 148 is very forgiving about rail to rail mis-trim (mistrim being other than perfectly flat rail to tail). Smaller and narrower boards are not.
For tjanulis..... What was the board you rode at Calema and how does the width compare to your Carve 145.
I suspect that both of you are not getting your board/rig fully perpendicular to the true wind when starting to get underway, and then to compound the problem you are not keeping the rig forward long enough to get the board fully moving before you add a bit of weight upwind of the centerline.
Keeping the rig upright (fore and aft here) or perhaps a little tipped forward or upwind a bit is not difficult, but I'd guess in your haste to get things going, you have passed over or skipped this very important skill.
So, try this.... work on getting the end of the boom straight downwind, and the rig balanced with the board fully in the "T" position right across the wind.
When you sheet in to start out, if the board starts to turn upwind, tip the rig a little more forward.
Be very careful with your footwork that you truly keep the board "flat" rail to rail.
Thie will help the board to plane off earlier.
As you move back progressively on the board, with your feet, try to keep the rig more forward to keep the board from turning upwind.
Do not try to "rake the rig back" until you are pretty much fully on a plane.
Once the board planes off, and you are in the footstraps and hooked in, THEN worry about raking the rig back.
I don't think either of you really have a rigging problem, or a mast foot positioning problem, but you've learned to leave out some very important "alignment" issues when getting underway.
Tjanulis...... where do you live.... anywhere near Cape Hatteras..... I'd love to see how you are rigging your sail and what your technique is.
You've really piqued my curiosity.
Hope this helps,