WordPress Admin Blank

I’ve been trying to log into the back end of my site, and it’s blank when I click on the standard tabs such as Dashboard, Posts, Pages, etc. The other tabs such as the plugins, store, etc. works fine though. The front end of the website works fine too. Been using wordpress for two years and this only surfaced recently.

A search online revealed that this is a common problem, and there were a number of suggested solutions, but I’ve tried them all and none of the suggested solutions work in my case.

The debug shows the following error message:

Strict Standards: Declaration of Walker_Category_Checklist::start_lvl() should be compatible with Walker::start_lvl(&$output) in …/template.php

I’ve already ensured there are no extra spaces at the end of the php ?> code and also in wp-config file. Really have no clue how to solve this issue.

I’m not very literate with coding. Desperately need urgent help. Very very much appreciate the help and advice in advance.