I managed to get it to work.
Not by knowledge but by pure chance I suppose.
First of all, I have to say that my computer is full of old Maniaplanet stuff, leftovers from all the betas that I installed years ago, etc.
The hard disk where the final installation was has died a year ago but there is still a lot of leftovers from other installs on the disks that are still working.
I also have several Maniaplanet data folders, in "c://users", in "e://documents" etc etc, filled with maps, skins, and that kind of stuff.
Now, what did I do to get it to work?
I bought TM Lagoon (really) and (probably for the above reason) it didn't install properly, after some hassle Maniaplanet began to install from scratch, asked me where i wanted the files.
When it was finished, I launched the launcher and it began to update to 4.1 (which it was already, probably somewhere else on the computer) and once it had launched it told me that my password was wrong (??) and I needed to reset it.
Once I got in the Maniaplanet Home, my packs were still all there but for some reason It needed to update them all (I chose HD).
Lagoon was nowhere to be found though...
I launched the Lagoon demo, it asked me for my product code and it worked.
I got back to Stadium, tried to open the crashing maps, but they weren't in there (I chose the wrong Maniaplanet personal files folder at install
) so I manually copied the tracks to the right folder (where I found a whole bunch of extremely old .gbx files)
Now, the game saw my newest maps, the ones that crashed the game, and I could open them all without the game crashing.
To be short, if you have problems, buy Lagoon (it is 50% discount!)
More seriously, I suppose that the installation has overwrote some corrupt file or repaired some faulty registry entry somewhere.
My advice to people having similar problems, try to reinstall a fresh copy of Maniaplanet, that might help.
