LNCS and thm-restate don't work together
If you import thm-restate and then define a new theorem (using either amsthm's newtheorem or LNCS' spnewtheorem), you'll get an error about a counter for that new theorem environment already being defined.
Obviously, this is not the case, because you just defined it for the first time.
The solution is to import thm-restate after you have defined your new theorem environments.