When you remove the Games\ArmaReforger\profile\.save\app1874880_xxxxxxxx\game\xxxxxx.Cain (or other map) it works again (for me)
When your stuck like this press escape once, then arrow up and enter twice (the menu options are hidden from view, and this helps to quit the mission).
I suspect the save files are corrupted or hanging on something.
This happened to me with and without mods (loading), while I think the saves were made using mods.