I have seen a similar issue before and it is most likely caused by too many failed logon attempts. Check what  your account lockout threshold is set to Administration console from Repository Options > Settings > Account Lockout Threshold. If temporarily disabling this setting resolves the issue then that is the cause. The next step would be to determine what is causing the failed logon attempts. In my experience I have found the issue to be related to web crawlers and bots trying to access the site.  There are ways to block bots from accessing WebLink. The first thing you should probably do is modify the robots.txt file if you have not already done so. see http://www.robotstxt.org/faq/prevent.html 

I may be able to provide additional information but I would need to know what the login page of WebLink looks like and how users are authenticating. Hope this helps!