Guests lose access to space that was previously working as expected

If nothing has changed in your setup and they are logging in from a work account it might be worth asking them to check whether their IT team has made any changes to their own setup recently. One issue we had previously, was that we had provided guest access to employees of a company whose own IT department blocked this somehow. Unfortunately I can't remember exactly the root cause of the issue (and what actually goes wrong might differ depending on whether the users come from a company that has their own Atlassian instance and therefore whether the guests have a pre-existing Atlassian account) but but, maybe something to do with the guest users Atlassian Admin restricting access to non-company instances?