All Links//references between cells in different sheets lost after opening a Libreoffice 5.x file in 6.x


I’ve a Calc Document for my home finances.
I have references beween sheets that used to work when I had Libreoffice 5.x.
After a long time not usid this Calc file I opened it again in Libreoffice 6.x (I upgraded to Ubuntu 18.04 from 14.04) and all Links//References are lost (I get #VALUE! as a result)

Can somebody help me … I imagine it might be a not so complicated problem

Thanks in advance

To complete the information I copy here one of the formulas within a Cell referencing to another sheet

=SUMPRODUCT(YEAR(‘Konto Auszug’.$C$2:$C$10726)=$A$6,MONTH(‘Konto Auszug’.$C$2:$C$10726)=$A$3,‘Konto Auszug’.$G$2:$G$10726=“rueckzahlung”,‘Konto Auszug’.$E$2:$E$10726)

Hope this helps to put complete the information

Probably a cell that is used in a numeric calculation contains text instead of a numeric value. But that’s just a guess, without seeing the document hard to tell. You can scrub the document of sensible information and attach it to your question.