Hey Suzanne,
We use Mac OS X as well and have been unable to reproduce this behaviour. Go to Customize > Portals > click Edit... next to the approrpiate portal and ensure that the Portal Name field is not blank. If it isn't then advise those customers to clear their browser history and try again.
Are you able to replicate this on your end? If so, please email me the Company Account name along with a number to reach you on.
- Ashur
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Ashur Shamon
Lead Solutions Specialist, Professional Services
Method Integration Inc.
www.method.me
Toll Free: 1.888.925.6238
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E-mail: a.shamon@method.me