If you save your prject in a higher version you cannot open the project in a lower version later. The version number is saved as a string within the file.
The reason for this is that some new features (or breaking changes) might be introduced in newer versions.
If you for sure know that you have not used any new functionality you can alter your files to switch back to the old version.
As mentioned, I reverted to 344.2 then tried to open a version saved with that version. That's the error message. If I try to open something newer, it won't let me. It says "This was saved with 350."
It won't let me open these files in 344.2. That's the issue. I can open files saved earlier than 344.2, but not those saved with it.
this is not due to r350 but the nwjs version being falsely marked as a trojan (https://github.com/nwjs/nw.js/issues/8090). You should be able to keep using 350, but be sure to export with an older nwjs version
I'm exporting with v71 in the nw.js as that's the only one that works with Steam. It's the same version I've been exporting with for 4 months without issue.