I got Plesk.7.5.5 on my windows 2003 box. By default MSDE 2003 was installed by Plesk and when the license key updated to manage MS SQL Server, everything was OK.
Unfortunately when I installed MS SQL SERVER 2000 sp4 I got the following error from mssqlmng.exe and also within Plesk admin panel:
Unable to update database parameters: Try to establish connection failed: cannot open database requested in login 'Master'. Login Failed.
Finally I found that I was installing MSSQLSERVER in Case Sensitive collation mode and the Plesk looking for 'Master' database which was named 'master' by default in MSSQLSERVER and can not find it.
I have uninstalled MSSQLSERVER and reinstall it in Case Insensitive collation mode and everything goes well.