The L2D Bug That Nearly Killed Me

I switched over because I really liked the 5.0 interface, and never in a million years did I expect something like this to happen to me.
No more preamble, here's the screenshot, along with how I fixed it. If anyone else runs into something similar,
I hope this post can help you out!
It really came out of nowhere. Mid-project, I went to save, and this notification popped up.
At first the notification only showed up occasionally, not that often,
but eventually it got to the point where I couldn't save anything new at all.
I asked around among friends who also use L2D, and none of them had ever run into this.
Then I dug through forum posts from various countries and found people with similar issues, but nothing I tried actually fixed it.
Finally I had a friend ask in a big international L2D group,
and the admin's suggestion was to roll back to version 4.2, so with a pretty anxious heart, I uninstalled 5.0 and installed 4.2 instead.
I was honestly terrified that a file made in the new version would break when opened in the old one Q But amazingly, the file saved just fine.
Later, another friend suggested maybe the file itself had gotten corrupted,
and that switching back to version 5 now might actually work fine.
So, figuring it couldn't hurt, I reinstalled version 5, and even more unbelievably, it actually just... worked???
So yeah, I burned an entire day dealing with this.
I was already behind schedule, and now this on top of it, I genuinely wanted to cry.
And this isn't even the first time I've run into an L2D bug...
I'm exhausted, honestly= =...
-A weary word from the God of Bugs-