Does it happen with any file or just specific files?
How do you know it's not a pb  with jOpenDocument
Could you provide the file? (after having sanitized it if needed https://wiki.documentfoundation.org/QA/Bugzilla/Sanitizing_Files_Before_Submission)

Created attachment 179172 [details]
example.ods

Here is an example file created with the newest stable version of calc that produces this issue.

Thank you for your feedback.
I gave a try with https://odfvalidator.org/, I've got:
The document is NOT conformant ODF1.3!
Details:
example.ods: Info: ODF version of root document: 1.3
internal:/schema/odf1.3/OpenDocument-v1.3-manifest-schema.rng: Info: parsed.
example.ods/META-INF/manifest.xml: Info: no errors, no warnings
example.ods/mimetype: Info: no errors, no warnings
example.ods: Info: Media Type: application/vnd.oasis.opendocument.spreadsheet
internal:/schema/odf1.3/OpenDocument-v1.3-schema.rng: Info: parsed.
example.ods/meta.xml: Info: Generator: LibreOffice/7.2.6.2$Windows_X86_64 LibreOffice_project/b0ec3a565991f7569a5a7f5d24fed7f52653d754
example.ods/meta.xml: Info: no errors, no warnings
example.ods/settings.xml: Info: no errors, no warnings
example.ods/styles.xml: Info: no errors, no warnings
example.ods/content.xml[2,9804]: Error: unexpected attribute "calcext:value-type"
e="string" calcext:value-type="string"><text:p>NUMMER</text:p></table:table-cel ----^ example.ods/content.xml[2,9996]: Error: unexpected attribute "calcext:value-type"
e="string" calcext:value-type="string"><text:p>foo</text:p></table:table-cell>< ----^ example.ods/content.xml[2,10789]: Error: unexpected attribute "calcext:value-type"
etc.

Weird, "calcext" is an extension but odfvalidator shouldn't complain since we use ODF 1.3 extended by default.

Eike/Michael: any thoughts here?

as usual, with the default LO setting of ODF Format version: "1.3 Extended (recommended)" use this validator flag:
-e: Check extended conformance (ODF 1.2 and 1.3 documents only)

or change the LO setting to "1.3" to omit extension attributes.

(In reply to Michael Stahl (allotropia) from comment #4)
> as usual, with the default LO setting of ODF Format version: "1.3 Extended
> (recommended)" use this validator flag:
> -e: Check extended conformance (ODF 1.2 and 1.3 documents only)
> 
> or change the LO setting to "1.3" to omit extension attributes.

Thank you for the hint, I changed the LO setting to 1.3 to omit extension attributes.

I would have thought that "1.3 Extended (recommended)" would indicate something in the ods file so https://odfvalidator.org/ doesn't complain about extension attributes.

I changed the version in Calc by going to Tools -> Options -> Load/Save -> General, adjusting the version from "1.3 extended" to "1.3" and saving anew. However, the issue persists. I uploaded a the file. It passes the validator but doesn't work with jOpenDocument.

Created attachment 179222 [details]
example-2.ods

odfvalidator is ok with 1.3, so it rather seems JOpenDocument pb.

Yes, seems like it