It could be a temporary thing with the server, but do some housekeeping if you haven't already - clear deleted items out and if other folders have much in from years ago those too.
If you have the installation disk you can refresh or fix problems, not sure how it's worded but it shouldn't change any settings or lose stuff to run that.
Ideally back up first, you should be doing this at least a couple of time a year anyway.
https://support.office.com/en-us/article/export-or-backup-email-contacts-and-calendar-to-an-outlook-pst-file-14252b52-3075-4e9b-be4e-ff9ef1068f91