Lately, I've been getting a whole lot of general exceptions that look like this:

AssemblyVersion: -1
PortalID: -1
PortalName:
UserID: -1
UserName:
ActiveTabID: -1
ActiveTabName:
RawURL:
AbsoluteURL:
AbsoluteURLReferrer:
UserAgent:
DefaultDataProvider:
ExceptionGUID:
InnerException: DataType "TextHtml" is not installed.
FileName:
FileLineNumber: -1
FileColumnNumber: -1
Method:
StackTrace:
Message: DotNetNuke.Modules.UserDefinedTable.UnknownDataTypeException: DataType "TextHtml" is not installed.
Source:
Server Name: GENERIC86

I've got Form and List installed (the most current stable version as of early April), which should have overtaken UserDefinedTable if I understand correctly, but I'm seeing this error over and over in my event log. The DataType sometimes changes — I often see otherwise-identical exceptions with "DataType "URL" is not installed," usually right alongside this TextHtml exception. What could be causing this trouble?

Only the site default, U.S. English.

I see them only in the event log. Whenever I'm working with it, FnL appears to behave flawlessly.