Vagelis

This is not an error message,this is a form and any visitor of your website can upload files using this form into your root folder.
Try to scan your website for malware and security issues asap as it looks like you may got hacked.

forgot to mention you need to remove this form as soon as possible.

It looks like you have the old url in your wordpress database
you can change this by using phpmyadmin ( you will need to change siteurl and home values inside wp_options table)
or you can try this (will be easier)

https://codex.wordpress.org/Changing_The_Site_URL

you website looks hacked to me,this long text is a double encoded function.
installing Wordfence Security plugin to scan your website would be a good start to solve your problem.

this is caused by a “broken” theme or plugin file.
since you edited your theme files i would upload the theme first and rename plugins folder back to normal.

you can change the line height by modifying 2px value to 4px etc