I am trying to modify an Excel 365 manifest file (to add a ribbon command as now required for MS AppSource) The file is accepted by the validator (validate-office-addin), but fails when sideloaded in Excel 365
Your add-in manifest is not valid.
(The previous manifest work here)
Sideloading in local Excel 16 (German) not possible, because there is no option to upload an Addin manifest (there is just "My Add-Ins") which open a web page.
Opening in "File > Options > Add-Ins > XML package" fails for the current and the old Add-In maninfest because ther is a problem with a security certificate.
Is there a way to get a more detailed error message?