Steps to reproduce:
1. I shortened the Roto Brush & Refine Edge Span in the Layer Viewer
Result: Error Message, but nothing else happened
Application: After Effects (Beta) v126.96.36.199
OS: macOS v10.15.7, RAM: 32.00 GB, CPUs (logical): 12
Interesting. I'm unable to reproduce based on steps. Do you encounter the error each time you adjust the span? Can you please provide a screen shot of your Layer Viewer prior to adjusting the span? Or a screen recoding would be even better. Thanks.
(Un)Fortunately I could not record any new errors. I will keep an eye on it, though.
OK. Thanks for the update.