There is a limitation in the journals entry browse of 32767 exchange rate records, so housekeeping needs to be performed as follows:
Click System then click Rate Maintenance.
You should manually delete unused exchange rates to free up space so there are less than 32767 exchange rate records. Deletion validates that the rate is not used against a transaction and also that there is at least one rate for each currency.
A possible initial approach might be to focus on currencies with few transactions where it is less likely that rates have been used.