Just wanted to close this out by saying all in all, I think v4.8.0 is pretty tight and stable. It's hard to argue with tighter integration of preferred locale information, probably as a leadup to getting some probably awesome native multilingual capability. And also hard to fault DNN core for not playing well with some other modules I had on the site.

When I look at the core and core upgrades, I am very appreciative and usually in awe at what has/is/and will be accomplished.

Penny Rand,
Before answering, I looked at your posts and it looks like you have db access and are tsql aware.

It's easy if you are only dealing with one locale, in the following case, I'll reference "en-US". If more than 1 locale, I'd be more than happy to help you further if you need it. Am also asuming that you are referencing a portal that used DataSprings Dynamic Registration? Although to be fair, it 's not at all a fault of DataSprings - they write great stuff.

1. the problem exhibits itself in this case when the userprofile table is missing preffered locale information for user(s) in the particular portal - the missing locale info will cause the error
2. if you're only using a single locale, it's easy - look in the ProfilePropertyDefinition table for the PortalID in question and the PropertyName caled PreferredLocale to find the PropertyDefinitionID.
3. look in the userprofile table for a profile in the same portal in question that has a userprofile defined and make note of the settings, especially the propertydefinitionid
4. first, clear the old data - delete from userprofile where propertydefinitionid = x
5. next, insert the new data to get everything back on track. in my case, I used
   "insert into userprofile
   (propertydefinitionid, userid, propertyvalue, visibility, lastupdateddate)
   select [your propertydefinitionid here ], userid, 'en-US', '2', getdate() from Users

Any questions, please ask.