I've apparently pinpointed the error's cause to a conflict with SandboxIE v2.85, specifically the sandbox control file (control.exe). avast! displays the error message only if the control.exe process is resident in memory; strangely enough, the main sandbox service (sbiesvc.exe) doesn't cause any problems.
I'll be posting this on the SandboxIE forums as well, and in the meantime hopefully Alwil and Ronen can get in touch with each other to see what's causing the conflict and hopefully fix it.