Hi all. We recently moved our App from .NET Framework 4.6.1 (on VB.Net and VS2015) to .NET Framework 4.8 with VS2019.
Now we have a little problem with a RDLC report using LookUp function and concatenating strings. Everything works well in preview, but when we try to export the report in PDF, concatenated fields result in #Error.
Report has a DataSet with 3 tables

Table "SchedeTecniche" has all the data of the report. Table "Diciture" has all the labels of the report in various languages. Both tables have a field "CodLin"; this field represent the language of the report data, and is used
(from a single SchedeTecniche row) to gather all the lables in the correct language from table Diciture.

Field "CodLin" is declared in RDLC file as follows in both tables
<Field Name="CodLin">
<DataField>CodLin</DataField>
<rd:TypeName>System.String</rd:TypeName>
</Field>
The report is a simple List Tablix which prints every row of the table SchedeTecniche. Problem is here with these two expressions:

Expression A is
=Lookup(Fields!CodLin.Value, Fields!CodLin.Value, Fields!Eti051.Value, "Diciture") & Fields!CodVar.Value
while expression B is
=Lookup(Fields!CodLin.Value, Fields!CodLin.Value, Fields!Eti052.Value, "Diciture") & Format(CDate(Fields!DatOraAgg.Value), "dd/MM/yy HH:mm")
All fields involved in the expression have a value, there is no Nothing (null) or DBNull involved. Everything is a String.
When I open the report in preview everything works:

But if I open print layout or try to export the report in PDF, Label text 051 and 052 result in #Error

Note that if I change expression A in
=Lookup(Fields!CodLin.Value, Fields!CodLin.Value, Fields!Eti051.Value, "Diciture")
removing the concatenation with Fields!CodVar.Value, everything works, even the error on expression B disappears.
So, I can understand that there could be an error in my expression A, but why an error in expression A also involves expression B? And, there is an error in expression A? I already tried using the .ToString() in every possible manner, using CStr() function,
adding a concatenation with an additional empty string and other fancy solution, but always the same result.
What can I do? What am I doing wrong?
Thanks in advance for every suggestion!!