That is strange.
I fired Xojo 2021r2.1 (minutes ago) and I get an update download proposal (it was a long time since the last time I saw that)…
Are-you sure Xojo 2023r2 was not downloaded/installed in the background (in your back) ?
I have commonly seen this type of project corruption with users who save to a synced folder using Dropbox or similar. If your projects reside in such a directory, I suggest you move your working copies elsewhere and copy periodically for backups.
I also don’t recommend using the binary format, but that’s limited by your license, as that format is a lot harder to recover from corruption issues.
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Well, new, i am also saving a copy of a project before working on it 
Thank you dear Anthony
I am only saving on my HD.
And well, for the binary…i am restricted by my license, yes.
If all else fails, I’d open a private case with support, attach the project file, and ask for help.
Dear Emily
When I open a project, always Xojo 2022 release 2 is shown.
Maybe 2023 was downloaded without my knowledge and now is running somewhere behind the Xojo 2022 release…
Has never happened to me in over 15 years.
Make backups , save often.
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You never know dear Jeff, yes, backups, saving maybe too…you never know what Windows or Xojo is doing behind your eyes
@Anthony
Dear Anthony, well i think it’s not such important in my case, but if you like and it could be important to me, i can send you the corrupted project file.
How can i get into a private case with support?
Hi Rolf,
I loaded a project with Xojo 2023r2 (developed / maintained with 2021r2.1) and saved it to disk under a new name.
The file (xojo binary), unchanged is larger than the “orgiginal”… I get a warning @ save time.
I loaded it with Xojo 2021r2.1, and get warning like what you wrote in your original message.
I run it, and it “worked” (scan a source folder and generate an html text file, then I asked to render that file in an HTLMLViewer) and that… “works”.
I do not know what to think, nor tell. (errors are so easy to do)
If you can watch with an hex editor your project, look at the first bytes:
RbBF …/… 2>‘BlokProj …/… PSIVStrn …/… 2023.02
If you have 2023.02 (or more…) you’ve done an error and forget about it. Else, the mystery depens. 
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You need to create the case in the Xojo tracker, add your project file, and select the private checkbox.
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Once you have created the case please contact Xojo Tech Support for help.
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Men don’t do backups! (But they cry often)
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When the backups are needed, and found to be blank… Been there…
I manage my backups myself, and I do them often, now from experience and habit!
I have tried in the past to use GIT. Rarely have I found something so abstruse to understand. Among other things, in one of the attempts, if I hadn’t previously made a manual backup copy, I would have destroyed years of code!
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You could try using Arbed to open your binary project and recover the new code that you added.
That can be done with the free version.
@Emile
Wel, i compared the hexfiles of the corrupted file and the backup file.
The corrupted file had some extra 37 lines in the beginning, all referring to a picture file in my project.
So I think, the project file wasn’t saved correctly.
Thank you very much for helping me
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@Ant Col
Well, it takes almost no time to save a copy of a project to a memory stick or something similar…
I have lost only 2 hours of coding, so no real big problem for me
@ Jeff
If you have multiple releases installed and you open a file, it will be allways opened with the LAST INSTALLED release, not with the one you saved it nor with the latest released.
Corrupted files when saving is common (At least in binary and in windows), when working with Xojo, you get used to lose some work now and then.
Save often, keep backups