This is since upgrading to the latest EE and MSM, all was fine before.
When I log into Site A I can work fine.
I then cross over, via the CP drop down, to Site B and can work fine.
When I try to cross back to Site A, via the CP drop down, I can’t.
I have checked and ensured that all admin and index files are set up correctly (but then it was working fine before ...)