This situation may occur for various reasons. The most frequent is that the maximum capacity of Outlook has been reached (2Gb of saved messages).
In order to verify if this is the reason, you must close Outlook and use the webmail for 1-2 days. If the same situation does not happen on the webmail, the problem is in Outlook.
If the question is not solved this way, please contact Plako through suporte@plako.net.