This was actually block variable error 91.

I've been having this now occasionally after upgrading to ACP v7. Didn't have any problems with ACP v6.
I've been connecting the telescope also from FocusMax by using ACP hub. I thought that this was required. This worked with v6 but now I don't know if that has been the reason for these errors.

Error usually occurs when ACP is doing filter offset adjustments when changing filter. I don't remember having this error during actual focus run. One time I remember that FocusMax just lost connection to ACP telescope hub but FocusMax continued to work for awhile. Then it crashed just before getting dawn flats.

I'm not using FocusMax acquire star. Just plain focus. I have TCF focuser and I'm using Temp Comp setting in FM.