The Merit Matters blog recently showcased some Fiddler Tips and Tricks . Why do you care? Because Fiddler is the troubleshooting tool that MS recommends for helping troubleshoot web client issues.
Dynamics GP: Web Client Wednesday – Fiddler Tips and Tricks from Merit Solutions
Archived post from 2013. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.