System has been illegally re-entered

In my app. I have a Datamodule and in it Session1 and DataBase1 with about
20 Tables and 4 Querys and Session2 and DataBase2 with 1 Query named
PrintQuery1. Sometimes if I open PrintQuery1 i get an Acces violation and
after it an EDBEngineError: >System has been illegally re-entered<. Adding
new SQL Statements and preparing PrintQuery1 works without problems.
This Error doesnt occur if after each closing off PrintQuery1 I disconnect
DataBase2 and close Session2.
All Tables I use are Paradox7 tables and each Session has another

Are there limitations on BDE or Pdox7, and why this error occurs only
sometimes ?

Thanks for help.
Werner Zuglauer