The entry is stated below for your reference.
Code doc currecny local currency
Cr. Vendor ABC 100 GBP 80 GBP
Cr. VAT 1 10 GBP 10 GBP
Dr. Expenses 2 90 GBP 70 GBP
Dr. Exh. diff. a/c 3 20 GBP
Since the local currency conversion didn't happen for the VAT GL code 1, the differential value was posted to the exchange rate difference account 3.
Issue here is local currency was not valuated for VAT GL , as shown above.
Note: There are successful postings for other dates, where the conversion has happened for the same VAT GL.
Regards