There is another error on windows. The assertion fails in MSVC9.0. And on MinGW, test app gives "SVN::Unknown error".
This error occurs after any attempt to "connect" or "checkout". I've traced the process and marked the point where it gives the error, though I didn't have the time to examine the situation. But hopefully it should give you a clue.
In SysExec.cpp (line 235):
I guess (only a guess) the error is probably due to a parsing issue.