DNG files cannot store xmp metadata

I use Adobe Bridge to manage my DNG photos, but noticed that I cannot add keywords to my photos' metadata recently. Every time I try to add keywords, it shows "The file cannot store XMP metadata. No changes will occur."

Around 3 weeks ago, I was able to modify DNG metadata in Bridge (don't know what's the version by then).

Files are with read & write permissions as usual. Purging cache in Bridge does not help.