Is the date matching for this rate table set to "Higher"? When set to higher, the rate date must be later than the transaction date. This could produce the scenario you are describing.

For example, the exchange rate was set on October 31. All transactions occurring before October 31st would use that exchange rate. However, any transactions after that date would produce the error message you're getting.

If this is the case for you, a solution would be to enter a new exchange rate using a date that has not yet passed. You should then be able to post the transaction fine in POA.