Following is the error screen –

If you all the while no problem on FreePBX recording, and out of sudden not able to retrieve the  recording and shows “DB Error: Unknow error” , this mostly due to DB table corruptions, this can be fix using following –

mysql -h localhost -u asteriskuser -p
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 7
Server version: 5.0.77 Source distribution

Type 'help;' or '\h' for help. Type '\c' to clear the buffer.

mysql>  USE asteriskcdrdb
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> optimize table cdr;

Done. You try again and it should work.