We now understand what's going on with the 'invalid signature' error that some people are seeing. Under some conditions, the ExBPA.Transport.xml file that was provided in the last v2.5 web update (~3 weeks ago) does not get overwritten by the v2.6 installer, causing an incompatibility. Not everyone will be affected by this, it really depends on the speed of the network connection. The repro is a little obscure, but if the last v2.5 web update took >1sec to download and store the 18Kb XML file, then you hit the problem with the v2.6 upgrade.
We have a fix for the v2.6 installer underway, and should testing complete successfully, we'll re-release the v2.6 MSI file. For anyone who's successfully upgraded already, there is no action to take, as the bug is in the installer, not the tool itself.
My sincere apologies to those people who have hit this problem. We certainly do take issues like this very seriously.
Kind Regards, Paul.