Why can't I download my program to an L3 series controller on Revision 20?

There is a known issue with Revision 20 of the CompactLogix L3 series firmware when multiple large I/O modules are in the rack. It is not limited to ProSoft modules, but any module with a large I/O size. So any combination of multiple ProSoft modules, or a single ProSoft module with other large I/O modules, can result in the occurrence of the issue.

Identifying the issue is easy; the first program download to the processor is successful but any subsequent downloads will fail.  Power cycling the rack will allow another program download, but once again any subsequent downloads will fail.  The workaround is to first inhibit all the large I/O modules in the project download the program to the processor, then once downloaded successfully, un-inhibiting the modules.

Rockwell has fixed this issue in Revision 21, as shown here at the bottom of page 11:
Revision 21 Change Notes

Update: February 2016 - Rockwell has released a new Revision 20 series firmware, 20.018, that also fixes the issue for version 20 users.