I have a farm with 2 front-ends using H/W NLB. Every few days lately,
the first server returns this for every site, DocLib, etc (except for
portal home, which works fine!) The second server never returns these
errors and has been up for months. If I IISReset on the affected
server, it works fine again. Recyling the AppPool does not.

ERROR

An unexpected error has occurred.

URL->Web Parts Maintenance Page: If you have permission, blah, blah

Troubleshoot issue with Windows SharePoint Services

url is https://portal.foo.bar/sites/anysite/_layouts/1033/error.aspx?
ErrorID=GenericWebPartError&ErrorIDArgs.....