Adding a copy of xmpd.dll didn't fix the problem. Strangely the LDCalc version on the computer that got updated from Windows 7 to 10 DID work. It turns out that it is version 0.1.0.3.
I copied that 0.1.0.3 version over the 0.1.2.1. LDCalc 0.1.0.3 came also with a xmpd.dll and a LDCalcIni.xml. It WORKED.
I substituted the LDCalc.exe 0.1.0.3 with LDCalc.exe 0.1.2.1. After confirming that I'd trust the prog I got a message that the LDCalcIni.xml (that came with 0.1.0.3) couldn't be read. After confirming I've got a running LDCalc.exe 0.1.2.1!!!
w.
I copied that 0.1.0.3 version over the 0.1.2.1. LDCalc 0.1.0.3 came also with a xmpd.dll and a LDCalcIni.xml. It WORKED.
I substituted the LDCalc.exe 0.1.0.3 with LDCalc.exe 0.1.2.1. After confirming that I'd trust the prog I got a message that the LDCalcIni.xml (that came with 0.1.0.3) couldn't be read. After confirming I've got a running LDCalc.exe 0.1.2.1!!!
w.
LEGO ergo sum