How about providing a list of the most common services, with the links hard-coded, and just as last option the possibility to enter a own, custom link:
Service: Tomboy Online
-> on OK, either authenticate, or directly ask for Custom Server URL or SD Card location.
Custom Server URL: <TextField> (just enabled, if radiobutton is on Custom Server)
Location on SD Card: <TextField> (just enabled, if radiobutton is on SD Card)
With this we narrow out typing problems when entering the link. Then, if we are not able to connect, it is probably the fault of the server. (if nobody can connect any more, we just change the link in the sources.)
Additionally, we have to change the error message to more different messages explaining what is wrong exactly.
What's your opinion on this?