Hello,
i created this thread to make the solution that @grotaclas found more visible.
The crash happens if you try to save/autosave and Portugal doesn't get the circumnavigation first

To fix this, follow these steps:
1. Navigate to: your drive\SteamLibrary\steamapps\common\Europa Universalis IV\events
2. copy the ExplorationEvents.txt
3. into: your drive\SteamLibrary\steamapps\workshop\content\236850\2968589224\events
4. Edit the ExplorationEvents.txt:
4.1. Search for: hidden_effect = {
4.2. Delete the entire hidden_effect
5. no crashes on saving anymore

thanks again to grotaclas who saved my mp game