For all interests I found the problem:
For ervery process a table is created in the database and a lot of variables are stored as longblob or longtext in this table. For every column only the first 768 bytes are saved in ervery column the rest of the bytes stored in another page. The background for this failure is that MySQL only can stored 8000 Bytes per row. When too much variables of type longblob saved the row size will to big and MySQL throw the error 139.
The size will be exceed when too much longblobs are written in a row. The size of one longblob isn't interessted because only 768Byte are written.
Not only document variables are stored as longblob all complex objects are stored as longblobs.
The workarounds are reduce size of variables, reuse the variables (not a good design!) or change your database!
Christoph
