Thank you for your post.
What version of FileMaker Server is now running on the hosting company's server?
Are you accessing the file with FileMaker Pro 12 or a later version?
If you are able to access the file with [Full Access] privileges, check the sort options for the portal in case this was disabled. If enabled, try sorting on a different field, save it, and then sort on the original field and save it. Does this work?
Thank you TSGal -- it actually ended up being a newbie error on my part, and not related at all to the server. I was sorting the portal rows based on a date modified field, but I had left the field type as "text" instead of "date." The sorting worked properly for months because it was sorting in descending order and 9 (September) would show before 8 (August) etc. The problem appeared when a portal row was entered in October (month 10) which fouled up the sorting because it was working on an alphanumeric system and 1 was before 9 as the first digit. Important lesson learned!