You’ve probably already covered the following points, but I offer them based on some limited experience and information I’ve come across.
The “This device cannot find enough free resources that it can use. (Code 12)” message is possibly indicating the same information as outlined under Linux in the first post of this thread - that BAR1 is not configured correctly.
Looking at Table 3, on page 3 here, it seems that BAR1 can be configured in either “Compute (default)” or “Graphics” mode.
Is it possible that the P100 you can successfully use is configured in “Graphics” mode and that the lower BAR1 size is able to be handled by your motherboard, where the “Compute” mode requirement cannot?
If so, perhaps you could get a vendor to reconfigure the card prior to shipping.
I had a brief look through your motherboad manual and could not find a BIOS setting that may allow BAR adjustment.
There’s a cautionary post along similar lines for the RTX A4000 here.