"The incoming variable library is invalid" but Illustrator itself generated that XML

I've been struggling with the Variables functionality in Adobe Illustrator 2024 (macOS).

I have 61 variables in Variables panel. I created an XML file by Variables > Save Variable Library... . But even without modifying that XML file, Illustrator rejects to load it back, saying "The incoming variable library is invalid." Obviously this is a bug in Illustrator, but is there any workaround for this issue? Where can I start debugging this?

I also tried writing a CSV file by myself and loading it, but it failed as "The filter cannot complete because of an unknown error." (What a useless error message!)

Any hints would be appreciated. Thanks.