I had a password which worked in this forum, but when I entered it in the fvdw-sl web UI under "System Management / Setup / Firmware Interface / User account for Plugout.net (Required for downloading)", I got the error message "Can't update the user account". Tried again to no avail.
Then I changed my forum password ⇒ problem solved. So it seems that the firmware cannot handle some valid characters.
These were, I think, all non alphanumeric characters in my former password: ,[?}=$\'#:~
(It was much longer, contained them in another order, and my new password and passwords used elsewhere are not related. So it should be ok to post this.)