Please check on the panel as reseller if the OpenDKIM key will be shown.
Login as reseller, click on Customer and then on OpenDKIM.
For each activated domain, there must be a valid OpenDKIM key.
If no key will be shown, then deactivate and activate again.
I also upgraded my system this week and I have seen that no valid key was in the table after the upgrade.
But I did not test mail sending, because I found the missing OpenDKIM keys right after the upgrade.
It is only an assumption.