To add, there is a corrosion problem:
"In all, Air Force maintainers are working on 17 access panels - as small as several inches and as large as two feet. Four of the panels on the topside of the aircraft have been found to be most susceptible to corrosion and will be replaced - at a cost of $50,000 per aircraft, not including labor. "
http://www.f-16.net/news_article2579.html
Although i still cant find any source claiming structural problems.