To clarify, I am not experiencing the symptoms. In an effort to troubleshoot, I am trying to reproduce it using the same equipment, firmware, OS, browser, and sample password settings:
Device: DIR-645
Firmware: 1.04B11 (12/04/13)
OS used to connect: Windows 7 Ultimate SP1
Browser used to connect: IE 11 build 11.0.9600.16476
Current Password: 123456789
Status: Able to login daily since Friday Jan. 8th 2014