Alright. Ran more tests tonight and again experienced the "oops Maxim" message and out of focus stars.

As this issue only arose with installing a new machine and the only difference in terms of software was the version of Maxim, I decided I would try a run using Maxim v 4.62. Focus between filters returned as expected.

Any ideas anyone as the a particular setting in the interaction between ACP/FMx and Maxim v4 vs v5 that may be causing this issue?