IBKR always says I don't have enough cash settled to make a trade. WHY?

This happened to me whenever I used the WebTrader. It would take 1-2 days for currency conversions to really happen.
If I do it via the User Portal or the Mobile App, it works almost instantly.

Or are you using the feature where you can buy x shares for a total price? That also doesn’t seem to work for me, because it always tries to reserve more money than I actually have to do that…