I think we had something like this before, where deleting the settings file solved the problem. However, to get some more insight about the problem, please download http://sagagames.de/stuff/mptrack.exe and check if it still happens with that version. If it still does, a notification should be shown saying "Debug information has been saved to ..." - navigate to that directory, and upload the .dmp file from that directory here (please .zip the file before uploading).

This one said that the plugin "decadance" failed to load, which was lately installed to my PC with FL Studio. I guess those 2 won't work together. After removing Deckadance it did work, but still shows a error that the Deckadance files are missing.

Ok, then you should check if the plugin is still registred with OpenMPT (View -> Plugin Manager) and remove it if it's still there. I guess that should solve the crash issues.

It didn't crash after the Deckadance remove, but it still gave a error. But it's fixed now, so this case is solved.

Which error was shown after removing the plugin?

A "Unable to find: vst.dll" error. But after that it went right into OpenMPT. Then i removed Deckadance and the next startup didn't show anything.