There is an anomaly in the Rockwell 1756-L1 (and others?) v34 controller.
As of this writing, controller v34.0 and v34.011 have it.
The symptom is difficulty in getting or staying online with Logix Designer.
The problem only appears when a program is loaded. For example, flashing is unaffected. Using either RSLinx Classic or Factory Talk Linx has the same effect.
Using RSLinx, the errors are 806-804535 or 701-804353 depending on whether you were online or not.
The solution is to open the Main Task properties > Configuration tab. Then check the box labeled “Disable automatic output processing…” You should then be able to download the program and stay connected.