Outlook is, it seems, not the best coded application. Various operations cause it to lock up when things don’t go as the software is expecting. I’ve also been getting an annoying issue where the fan on my laptop will come on. I traced this down to outlook.exe consuming about 30% CPU (this is on an i7-5600U).
Eventually I discovered that if opened the Free/Busy Options for my calendar, the CPU dropped off. But only if I went via the following route into the dialogue box: File – Options – Calendar, then clicked the Free/Busy Options button. Whilst that dialogue was open, the CPU dropped right off to almost nothing. (I subsequently discovered that opening other dialogue boxes from that page had the same effect). However, whilst I was in the Free/Busy Options dialogue box I noticed an account that I had given permissions to was showing with the format of an NT SID, i.e. S-1-5-21-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxx-xxxx. I removed that account from the permissions list, but it made no difference to Outlook’s misbehaving.
I also had that person’s calendar added under My Calendars – though that was still showing with their name. I removed that calendar and the problem seems to have gone away.
This situation occurred because the person left the organisation, and can only assume Outlook was still trying to access their calendar, even though it wasn’t ticked under My Calendars.