By default, all LW 5.5 versions run as Basic which only connects to 3 UP's, not Pro which connects to 5 UP's. Only way to get it to connect as pro would be (1) add the pro registration number & (2a) turn the computer system date back to 2010 until LW has opened. But changing the date when LW is connecting will throw all connections off temporarily. (2b) alternatively run your system in 2010 whilst LW is open.

Pro registration for 5.5 only ran for 12 months. I suspect the registration number is date based.

If I recall correctly, you were a previous 5.5 pro user so probably already have the pro registration file in your settings folder.

The LW company made the 5.5 pro versions work like this in an attempt to stop scam sites downloading a copy of the pro version & re-selling it. Scam sites were a big problem in those days, since they advertised via google so their ads would show whenever anybody googled limewire.