Yuri,
It's been a long time since I've used J indirectly like this, but I believe the case was:
JErrorTextM will almost always return "Busy with previous input". This result is fairly meaningless (on par with the output of a C compiler when you forget a semicolon or brace).
It simply indicates your application has broken (and may be suspended, as with 13!:0]1 ? I forget).
If you want more detailed insight into the problem, inspect the result of the previous call to JDo. Zero indicates success, non-zero indicates failure, and is similar to an index into 9!:8'' (J's list of error messages).
I say "similar to" because you must subtract one first (to account for the fact that zero indicates success and therefore can't also indicate failure of the type 0{9!:8'' ). If you like, you can think of the result of JDo as an index into (<'OK'),9!:8 '' (or as <:13!:11'').
Obviously, 9!:8'' can give you the type of the error, but not its location. For that information, try 13!:12'' . That will sometimes contain the name of and line number in the entity which broke (usually a named verb).