Hello,
Unable to access the menu
I have the following error message:

"Retrieving styles data failed!"

Hi there,
please more details. Which extensions are installed?
>> If "joomsocial" is installed please disable the joomsocial updater.
>> If Gantry was installed before, please remove all gantry related plugins and extensions

And please make sure that all files and template folder is writable.

/administrator/templates/system - 777
templates/jp-{your template} -777
www/templates/jp-{your template}/less/style.less -777

Hello,
Great, problem solved. This came from Gantry. I uninstalled everything and now it works.
Thank you