After restarting eM Client, it does not remember the previously configured sizes of the left pane (Mail/Calendar/Tasks/Contacts pane), the right pane (Contact Details/Agenda/Invitations/Chat), or the reading pane. I customized the sizes of these panes by dragging their edges, but after a restart eM Client reverts back to default sizes.
You could check that there are not any instances of eM Client left running in the task manage after closing.
Another option to try is to start eM Client with a blank database, and see if the issue is still there. To do that first make a backup of eM Client (Menu > File > Backup) then close eM Client and delete the *C:\Users* _ yourusername _ \AppData\Roaming\eM Client folder. You may need to enable hidden items in Windows Explorer to get there. If that does not make any difference you can restore your old database (Menu > File > Restore).