Multiple threads and Sqlite - database disk image is malformed

When I try to read (select) from sqlite database from ~1000 processes i keep getting errors like

SQL error: database disk image is malformed

or less often

SQL error: file is encrypted or is not a database

from some of threads.

I test this program & db on 3 hosts:

1. with older 1-core CPU - no problems, no errors
2. with 1-core VPS - errors occur
3. with 8-core i7 - errors occur

DB is always in /tmp/ folder so it's basically held in RAM. Integrity check says DB is fine.

What can cause this problem and is there a solution?