There are many reasons why reaching out to support will be more effective. Logging may be tied to accounts, support systems may upload client side logs where appropriate, an event may be tracked to determine a point in time when you had the problem so relevant logs can be checked.

Their messaging probably could have been better here to say "we're looking into this. If you have time, please file a support request".

On Fastmail.com, the support request is two clicks away, under the big ? button in the upper right and "Contact Support". I'm not sure how much time you have but it took me less than 1 minute to find this button and submit from my logged in account.