Re: recent master: Missing '\end_inset' of Float inset

[PhilipPirrip]

Just noticed the same. It seems enough to have "\lyxformat 543" changed 
to, say, "\lyxformat 534" in an otherwise regular, simplest, empty LyX 
2.3 file.

Thus:
save an empty file newfile1.lyx
change lyxformat number in it
the warning will show when reopening