CPU1/CPU2 has an internal error (IERR)

Have you checked the power supply (s)?  That is the most likely candidate to cause internal CPU errors. Consider replacing the supplies

Is there a way I can confirm the supply(s) are at fault?  I am currently rebuilding my drives to a single RAID5 volume and will try to install OS afterwards.

Boards, Disks, Accessories will not cause an internal CPU error.

It could be a unique motherboard error, but this seems less likely than Power Supplies.

Filtering, voltages, capacitors can all go bad in a Power Supply.  

I don't know any way to conclusively test for this however.

How old is the server?   Worth repairing?

It is a while back, but I did have a workstation with a CPU error and after stripping it down and testing, the vendor said Power Supply. Replacing it fixed the issue.

Again, how old is the server before investing in it. You may wish (if worth it) to get it professionally repaired with a short term warranty on the repair.

Run the system's internal diagnostics and see what it thinks about itself.  The Dell diagnostics are comprehensive and test everything that can be tested, including power supply voltages and memory.  You may want to skip the disk diagnostics if the system uses a RAID controller from the primary drive.

There is nothing fatally wrong with this system; if there were, it would not boot at all and it would be showing error LEDs instead.

https://www.dell.com/support/article/us/en/19/sln283546/how-to-run-hardware-diagnostics-on-your-poweredge-server?lang=en

If the system fails its own diagnostics, it will return an error code indicating the type of problem and the source.

If the system passes its own diagnostics, then it may be a software issue and not hardware.

Let us know what you discover.

Side note:  There are some BIOS updates in the Poweredge family which cure ills vaguely like this.  Check the BIOS revision level installed on the server, check at Dell's web site to see what the most recent level is, but if there is a more recent BIOS do not apply it until other possibilities have been considered.

There may be cpu stepping setting in the bios that may need to be turned off, but windows 2012 should be able to handle different cpu settings.

https://www.dell.com/support/article/us/en/04/sln298205/dell-poweredge-resolve-cpu-1-or-2-has-an-internal-error-cpu-ierr-on-servers?lang=en

How have the system's components changed?

Addon cards, network, hba?

It's trivial to eliminate the PSUs, try with one then the other. They plug straight into the mobo so there's no internal PDU.

Since the above doc suggests the OS to be the cause it could just be a corrupt Windows DVD.