Further evidence.
Hoo_man says: «If you have a space in you name (like I have), don't make a _ as replacement and it works. Eg. http://strategy.wikimedia.org/w/index.php?title=Special%3AUserRights&user=Hoo+man works http://strategy.wikimedia.org/w/index.php?title=Special%3AUserRights&user=Hoo_man doesn't work».
Leinad says that it doesn't work for some sysops on pl.wiki, neither.

Another old example: http://pl.wikipedia.org/w/index.php?title=Specjalna%3ARejestr&type=rights&user=&page=User%3ALukasz_Lukomski&uselang=en (username with underscore, but it worked after two weeks).
http://meta.wikimedia.org/w/index.php?title=Special%3ALog&type=rights&user=&page=User%3ADferg+%28test%29 didn't work.

(In reply to comment #2)

http://meta.wikimedia.org/w/index.php?title=Special%3ALog&type=rights&user=&page=User%3ADferg+%28test%29
didn't work.

Forget it, he didn't try because he forgot password. Sorry.

See logs: problem had disappeared on Meta three hours later.
But, as hoo said, it still won't work if you type the username with the underscore; and after saving user groups the space is converted to underscore and you're not able to modify flood flag again unless you re-type the usernam with the space.

(In reply to comment #4)

See logs: problem had disappeared on Meta three hours later.
But, as hoo said, it still won't work if you type the username with the
underscore; and after saving user groups the space is converted to underscore
and you're not able to modify flood flag again unless you re-type the usernam
with the space.

I believe that issue was fixed in r66258 (however I don't think that fix is deployed).

(In reply to comment #5)

(In reply to comment #4)

See logs: problem had disappeared on Meta three hours later.
But, as hoo said, it still won't work if you type the username with the
underscore; and after saving user groups the space is converted to underscore
and you're not able to modify flood flag again unless you re-type the usernam
with the space.

I believe that issue was fixed in r66258 (however I don't think that fix is
deployed).

Thank you.
No, it's not deployed.
Comments on the rev comment: this bug is not about lowercase only; I don't think that it can be a regression from r57064 because the pl.wiki example above came 26 days before, when the last relevant deployed rev was r54429.

WMF wikis are still at r64689, if I understand correctly; on zh.wiki they're experiencing some problems which should depend on this bug, see bug 26117 comment 13 for a more detailed description and a test.

(In reply to comment #7)

WMF wikis are still at r64689, if I understand correctly; on zh.wiki they're
experiencing some problems which should depend on this bug, see bug 26117
comment 13 for a more detailed description and a test.

The fix i mentioned earlier only fixed issues if you type your name in the wrong way. In all cases if you type your name in not using underscores, and proper capitalized, it should work fine (and if it doesn't its a different issue from the issue fixed by r66258 )

Meta-Wiki has been updated and now this seems to be fixed.
However, I experienced a similar problem with block interface: I couldn't unblock a user because I received a "block ID not found" error, when I followed the link on the block log (http://meta.wikimedia.org/w/index.php?title=Special:Log&type=block&page=User%3AMeta-Wiki+Welcome) i.e. http://meta.wikimedia.org/w/index.php?title=Special:BlockList&action=unblock&ip=Meta-Wiki_Welcome with the underscore, but it worked when I used the link on [[m:Special:Block/Meta-Wiki_Welcome]] (which is not affected) i.e. http://meta.wikimedia.org/w/index.php?title=Special:BlockList&action=unblock&ip=Meta-Wiki+Welcome with + .
I don't know if there are other cases.

Still an issue?