The most problems occurs to the usage of plugins. The first step should also be to deactivate all plugins and see if the error(s) was made by one of the plugins you use. Then, activate step by step the plugins and after each plugin activation, take a look at the problem. As soon the problem occur again, you’ve found the problem / plugin with the problem.
Due to the fact that more than 95% of all problems our clients tell us are caused by plugins, this should always be the first step for your trouble shooting.