OK, I guess I figured out what had caused the problem: the Plone 5.2 site I'm working on is supposed to replace an old CPS site and I want to have at least some URLs of the new site to be the same as of the old one, especially for personal pages of some members.
I noticed the .../@@manage-aliases action that provides an "easy way to make an item available under two different URLs". But, because it is also written that "To change the primary url of content, use Actions > Rename", I decided to rename Members to members. And this broke the "My Folder" user action feature.
IMHO, renaming sensible folders either should be impossible or, at least, when they are about doing so, administrators should be warned this may break some Plone's features.