This is what I did when I ran into a problem installing XM8 27.3 last week. First, as I was using a brand-new laptop, I wiped the drive and reinstalled from the restore partition, which is on a second drive in the same machine (I wanted to make sure a full restore back to factory settings would work before I had to do it on a deployment, anyway). This full restore removed any lingering/hidden XM8 files which can interfere with a successful installation.
I then started a fresh install of 27.3 and proceeded as far as I could.
In a Word document, I described everything I had done up to this point (including verbatim any error messages) and logged into Xactware's online help chat. One needs a user name and password to do this; you may wish to call Xactware sales (801 764 5900, for sales press "2") to obtain these in advance.
Once logged in, I pasted my Word description to the chat window. The online tech ultimately deferred to a co-worker who remotely took over control of my laptop's desktop and easily fixed the problem.
You may wish to request in advance that the tech's actions be performed slowly enough that you can follow them, as in my case it went too swiftly for me to learn anything.
Note: Online chat support is free, as opposed to telephone support, which costs $20 a pop after the first instance. I have been quite satisfied with the online help I have received over the years; your mileage may vary, but your overall satisfaction may very well depend on how succinct and organized is your presentation.