BDE 4.51, BLOB already open

I've got the following installation for my Application:

- D 3.0
- BDE 4.51
- Alias: DBase-Native-Driver, (Blob-Size 1024)

I always get the Error "BLOB already open", when edit (by  setting) any
Memo/Blob-Field after having made a Lookup (self made).

TBlobField.AsString := NewStringValue         >>>>> BLOB already Open

Only when I've posted the record, I can edit this fields.

I've debugged now already for a long time in my Source-Code, but didn't find
any reason for this event.
I don't see any reasons for this error:
- The table is only opened once
- The record is not locked by an other Client
- The table is not corrupt (already recreated for several times)

So what should I do now ? - Should I use any misterious Dbi-Orders
(DbiFreeBlob) to close the
Blob-Field, but how ?

please answer me, I need your help