Strange invalid transaction handle Error

[sugi]

VCLs Settings :

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/* Connection Settings*/
ConnDba->Pooling = true;
ConnDba->PoolingOptions->Validate = true;
ConnDba->Options->LocalFailover = true;
ConnDba->Options->DisconnectedMode = true;

/* TIBCQuery Seetings*/
T_JRNL1->CacheUpdates = true;
T_JRNL2->CacheUpdates = true;

Database location is on remote server, connected via Internet.
When executed this code :

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void __fastcall TInpBnkInOtFrm::OpnJrn1Done(TObject *Sender)
{
   if(T_JRNL1->IsEmpty()) T_JRNL1->Append();

   OpnDt *Dt4 = new OpnDt(true);
   Dt4->Dt = T_JRNL2;
   Dt4->Id = T_JRNL1JRNL1ID->AsInteger;
   Dt4->Resume();
   Dt4->OnTerminate = OpnJrn2Done;
}
//---------------------------------------------------------------------------

Sometime, runs OK, but mostly got Invalid transaction handle Error.

That codes, basically open an Query on another thread, here is the thread code :

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__fastcall OpnDt::OpnDt(bool CreateSuspended)
   : TThread(CreateSuspended)
{
   FreeOnTerminate = true;
}
//---------------------------------------------------------------------------
void __fastcall OpnDt::Execute()
{
   Dt->Close();
   Dt->ParamByName("id")->Value = Id;
   Dt->Open();
}
//---------------------------------------------------------------------------

If I pause thread execution for a second, changed codes to this :

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void __fastcall TInpBnkInOtFrm::OpnJrn1Done(TObject *Sender)
{
   if(T_JRNL1->IsEmpty()) T_JRNL1->Append();
   PauseInSec(1); -> this will pause app for a second
   OpnDt *Dt4 = new OpnDt(true);
   Dt4->Dt = T_JRNL2;
   Dt4->Id = T_JRNL1JRNL1ID->AsInteger;
   Dt4->Resume();
   Dt4->OnTerminate = OpnJrn2Done;
}
//---------------------------------------------------------------------------

App runs OK. Why ?

[ViktorV]

IBDAC is thread-safe, but the restriction is that you cannot use one connection (the TIBCConnection component) in several threads, and you should use a separate connection in each thread.