Under some circumstances (sorry, I don't have minimal code to reproduce the error) named parameters in VarPyth not cleared between function calls, and function called without named parameters may recieve parameters from previous call with named parameters. Condition check in line 1091 of VarPyth.pas seems to be excessive; in my case commenting it out solved the problem:
{if CallDesc^.NamedArgCount > 0 then }GetNamedParams;