[Exchange Web Services] Email sending failed due to the following error: An internal server error

Hello,

we have multiple Office365 acounts, all users work with eMclient. As of today, one account suddenly starts getting error when trying to send email:
[Exchange Web Services] Odesílání emailu selhalo kvůli následující chybě: An internal server error occurred. The operation failed., The value is set to null
Parameter name: options.ImceaEncapsulationDomain

Tried to remove and reenter the account, tried the new version with airsync and no result. still the same error, only for this one account. Can you specify, what exactly the error means and if there is a way to correct it? It is driving us mad… Thank you…