When trying to post a reciept, having too many decimals on the exchange rate causes a silent failure – that is, when you press POST, the page simply reloads, as if you'd pushed the update button.
So, inputting 0.163799546 doesn't work, but 0.163799 does. This might be a validation error, but since there's no feedback (and nothing in the logs either), it is exceedingly hard to realise what to do to work around it.
I can't find an edit button here, but this seems to be wrong. Instead, I have a “cursed” invoice that I can't create receipts for, no matter what I do. It just reloads the page, no matter how I diddle the exchange rate :(
I think this was corrected in 1.3.42 and is caused instead by rounding issues after the exchange rate is applied.