"Value cannot be null. Parameter name: key" when aligning docs

Hi all!

I am trying to do my very first doc alignment on Trados 2021. It seems it correctly select the (new too) translation memory, upload the two docs, carry out the alignment but finally fails to open the alignment file and says "Value cannot be null. Parameter name: key".

Am I doing something wrong?

Thank you very much in advance,

Paola