I have posted this in the MS support forum but there has been no response so I thought I'd try here in the hope of getting some ideas.

Further to the description, below, Outlook only runs out of resources when there's also a fairly large Excel workbook open (any one of several , it isn't specific to a particular Excel workbook). Excel itself never reports a lack of resources, but closing the Excel workbook makes Outlook happy again !

Problem description

I have a straightforward installation of Office 365 on Windows 11. Usually there's not much else running except Google Chrome and sometimes a large Excel workbook or small-ish Word documents or a photo editor.

6 months ago I started getting messages that Outlook had run out of shared resources and that I should close any messaging applications.

I wan't able to fix it that and had to restart Windows to get Outlook going again.

Then the problem went away - I am not aware of any system or application changes around that time.

A week ago it came back again and now it forces me to do a restart every few hours, sometimes more often, which is a real pain.

How can I set about diagnosing the cause ?

The only relevant thing I have thought of is to check RAM usage but I always seem to have at least 7GB free out of 16GB in total.

When there isn't the resource problem, Outlook is behaving just fine.