Hello,

All of a sudden users are being signed in as other people's accounts and with the ability to read private messages and such. Could somebody help asap on this please/

I did notice the following, too.  

If users are signing into other peoples accounts, then sessions are being cached on your server. Whatever caching is in place on your server needs correcting so it does not cache sessions, or disabling.

all sorted now. Thanks for the quick response.

No problem at all. Glad you managed to get it sorted out