This happens to me every couple of months… going on twice though this month.
The message is useless, as we cannot save back anymore from 2025, etc.
I end up copying everything out, saving a new file, then opening the previous file, hoping that the last recent save has my scenes, etc. in place - then pasting back in my model, section cuts, etc.
It is a PITA.
It is a pain, and the message is particularly infuriating as it recommends a course of action that has been intentionally removed by the developers. It’s a truly kafkaesque level of gaslighting to remove a function and then recommend its use, good joke on the user. The message text should be changed. Copy/paste is the best method I know of recovering from this.
However there is an extension that adds back the ability to save to previous versions.
I know of the extension but have never installed it… does it get you around this issue?
I just lost all my scene setups, etc…
I have not tested it in 25, but it functions in 24 to allow saving to previous versions. If this clears the issue with a given file is a dice roll. Certainly saving to 20 or earlier will loose tag folder structure as they did not exist.
You might try copy paste everything to save the geometry then delete all geometry, purge and save the empty offending file with no geometry to preserve a blank file with scene info? Then paste back in?
I’ve been doing that now, but often I lose the connection to the active section cuts… I could live without the Tag folders if at least it would save out using the extension - but I’ve had no luck with ‘Save A Copy…’ - so not sure if this would work. Will load it and try it next time this happens.
@dezmo’s extension takes advantage of the fact that while the devs have removed from the GUI the ability to save as an older version, they have not (yet) removed it from the Ruby API. Since it is not clear what is causing the failure to save in the first place, only by trying the extension can you find out if that is a usable workaround.
I won’t speculate on why they did this; I don’t know.
Hot off the presses, I have 0 knowledge about this, use at your own risk
What @slbaumgartner said is exactly true. So my extension “save”, will be as good as SU is. 
Therefore to keep the real visibility of the tags for earlier version than SU2021, determined by its folder - the snippet what @endlessfix linked can be used (but I did not tried).
One more thing:
If you select v_2021 as the save version, you should know that it will mean the current (actual ) version. (VERSION_2021 is the “highest” version number constant that Ruby API can handle.)
So the file will be saved to the current version of Sketchup (SU2025 in this case), but can be opened in SU2021, certainly.
