Most probably this is a database issue.
On a PC, close eM Client and open a Windows Command Prompt. Not a powershell, but a command prompt. Paste in this command including the leading "
"C:\Program Files (x86)\eM Client\MailClient.exe" /dbrepair
Or, on a Mac, quit eM Client and open a Terminal. Paste in this command:
/Applications/eM\ Client.app/Contents/MacOS/eM\ Client /dbrepair
When that is done, restart eM Client as see if you still have the issue.