Hello,
In version 4.0.6 I have fixed a bug related to fixer.io exchange rates that I believe was causing your issue. After upgrading, please wait for up to one hour until the exchange rates from fixer.io refresh. (Or, if you have wp-cli installed, you can trigger an instant refresh with wp transient delete wallets_rates_last_run).
If the issue remains after that, please let me know which exchange rate providers you have enabled, and I will look at this again.
with regards