211 (sysprocplan) error

I had created 6 stored procedures last November to do some nightly batch work and have had no issues with them until 1 of them last week started failing nightly with a 211, then 154 error. I had just read the thread on 211 errors and started updating stats specifically for these 6 SP's (even though only 1 fails) daily as well, but I still am getting this error. The tables referenced by this SP have not changed in months and we run a "update stats medium" every night (2:30 am) that completes successfully around 4:15am. I run my update stats for the SP's at noon every day and the SP executes at 12:25 am daily and fails immediately with the 211 error. What am I missing here? TIA, Randy