This does appear to be the same issue that @DavidRGreen reported earlier. In that report, the conditional formatting test worked in version 9.3.1.8, but in version 9.4.0.129 the same scenario started producing an error, and the case was added to our internal bug tracker for investigation.
Operational issues with spreadsheet on version 9.4.0.129 (x64 exe)
We will continue working on this bug and will update you here as soon as we have more information.
In parallel, we will also review the other spreadsheet behaviors you described (conditional formatting on N2/M2, recalculation of the hidden AB/AC/... columns, and dashboard updates after changing the x markers) to identify the cause of these issues.