Make sure you are always using the latest Merlin, Merlin Server, and Merlin iPhone app versions. Update your software if necessary. Merlin and Merlin Server can be found here
The latest Merlin iPhone app can get downloaded over the iTunes App Store
For Web access: Make sure you are using a supported web browser as stated here
Check other firewall settings in your system, maybe in your router, and adjust accordingly.
In case you are accessing the Merlin project over the internet, make sure 'Port forwarding' is enabled in Merlin, 'Forward in Router' is enabled in your Merlin Server or manually on your router. You will find some tips on various router settings here. To work around problems caused by the dynamic IP address of the router, please read our Merlin How To tip.
If you can connect to the server, but do not see any projects listed, check whether the security setting 'Hide from anonymous users' in your Merlin projects is enabled by mistake. If this option is necessary, there is nothing to trouble shoot. Simply authenticate yourself as the specified 'user' resource defined in the project. Read more about it here