Error tracing problem

I have a query in the PQE. All column stats look something like this, i.e. zero errors:

In the PQE grid, the data looks how I expect it to look. Some blanks and some values. No errors.

When I copy the table and paste it into Excel, it refuses to paste and spits out an error file full of this:

XML ERROR in Table
REASON: Bad Value
GROUP: Cell
TAG: Data
VALUE: null

When I try to apply my queries to the report, I get the following errors:

Expression.Error: The type of the value does not match the type of the column.
Details:
    07/10/2022 00:00:00

I suspect this all has something to do with the 'New Query' routine initially identifying this column as text type from the first 200 rows, when it is in fact datetime type. (The first 200 rows were all blank.) I changed the type afterwards, but now the column's type seems to be broken in some mysterious way. I say 'mysterious', because it is clearly working fine in the PQE grid.

I realise it might be quickest just to delete everything and start again, but do you have any suggestions for how to trace this error and to understand where it is going wrong?