When you cannot login any more in Testlink because of the following error:
Bad Request
Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.
Cookie
it means that some of the cookies used by Testlink have grown too big for a normal HTTP header.
If you are using Apache, it is possible to configure the server to accept bigger header filed sizes by using the following directive in apache.cong (or in your vhost definition):
LimitRequestFieldsize 1048576
(default value is 8K)
Testlink project has an open issue for this in their mantis bug tracker: http://mantis.testlink.org/view.php?id=3714
And it has been fixed for versions >= 1.9.20 by this commit: https://github.com/TestLinkOpenSourceTRMS/testlink-code/commit/12d29d12d3ea82ac2f9746f4d96b36af817d32a8 (I haven't tested it yet)