Strange error message at opening epub in Sigil

[Leonatus]

Windows 10, 64 bit (another computer with 32 bit), Sigil v. 1.4.3 LibreOffice v. 7.0.4.2 Writer2epub v. 1.1.28.

I'm very satisfied with the plugin; it works super-reliable.
But there is one issue appearing steadily when I want to open a epub created by the tool in Sigil. The message is - more or less, for I have to translate it:
"This epub contains not correctly built HTML files or files with missing DOCTYPE, html, head or body elemts. Sigil can try to repair those files automatically which may in few cases lead to loss of data."

When I click "o.k.", everything is fine and I can work easily with it.
But, as this messages occurs always with epubs built with the tool, I wonder what kind of errors might that be, and if it were possible to avoid them from scratch.

Is this a known issue?

[Doitsu]

Originally Posted by Leonatus

The message is - more or less, for I have to translate it:
"This EPUB has HTML files that are not well formed or are missing a DOCTYPE, html, head, or body elements. Sigil can automatically fix these files, although this may result in minor data loss in extreme circumstances."

This has nothing to do with the Writer2Epub. A couple of versions ago, KevinH added a missing doctype check, because HTML files without doctypes are technically invalid, even though EPUBCheck won't report this as an error.
I.e., you'll get the same error message for any number of books created by totally different apps.

[Leonatus]

Ah, thank you! So, there is nothing I can/should do before creating the epub, supposingly?

[Doitsu]

Originally Posted by Leonatus

Ah, thank you! So, there is nothing I can/should do before creating the epub, supposingly?

Unfortunately, there's nothing you can do.

[Leonatus]

Sorry, I should open a new thread!