'pcie Installed In Slots 3 And 4 With A Single Cpu' But Slot 3 Is Not In Use

Dear geeks

I am changing the computer on a complex machine. The 'new' computer is a power beast running on Win7 that I am trying to recycle from another complex machine or at least it was a power beast a few years ago.
I have transferred all the cards from one PC to the other and all went well. When starting, I first get the following message before Win7 boots:
'Pcie installed in slots 3 and 4 with a single cpu. Then i press F1 and windows boots normally.'
If I press F1, Win7 boots as normal and as far as I can tell, there are no problems.
I would like to upgrade to Win10 and unfortunately the install fails at the very end. We get an error message.
We are now trying to do a clean install to see if that works.
My question is: why do we get the error message about PCIe slots 3 and 4 from the bios since slot 4 is actually not in use! How can we get rid of this in case this is what blocks the Win10 installation?
Thanks for your insight.