You'll notice
Useful2 Signal1
Marketplace names in settings must be plain directory names or they're refused.
What's wrong with this entry?
Anonymous. No account, no email.
What
A plugin marketplace declared in your settings has its name checked before it is used as a cache directory. Names containing path separators, ending in .git, empty, or otherwise not a normalized plain name are refused rather than written to disk. The check applies to every settings-declared marketplace.
Evidence
is not a plain directory name; refusing to cache it
Strings lifted out of the shipped bundle, so the claim above can be checked against them.