In the last few days we have seen that a few of the MT routers are not
accessible through Winbox. They can be accessed via Telnet/FTP
normally. But when we try to access them from Winbox it shows
routeros.dll not a valid windows image and missing RouterOS winbox
plugins.
It is happening from all our PC's and from any network. We have tried
rebooing the MT routers but nothing changed.
Any suggestions, what might have happened. The versions are 2.8.9 and 2.8.11

you answered your question by yourself with the last sentence. i would suggest to upgrade and thensee what happens