Fixing Drupal Website Issue

Dears, we need someone to fix our Drupal website ([login to view URL]) issue described as below, Symptoms: 1-unable to login to the admin section. 2-some pages are down such as and not limited to [login to view URL] Technical Points: to the best of our knowledge and previous checks which you may find additional issues, lease find the problem statement below: [09-Apr-2017 15:43:49 UTC] PHP Fatal error: Interface 'Psr\Log\LoggerInterface' not found in /home/iyounis/public_html/core/modules/syslog/src/Logger/[login to view URL] on line 13 the above mentioned is the first instance of the error, This error is coming from 9 Apr on wards. When we researched on the possible issue, We have found that some plugin might have been installed at that time which is creating this issue. We will need to remove that plugin. Actual Needs and Delivrables: 1- Fix the login issue. 2- Update all the plugins. 3- Update the Drupal core. Acceptance Criteria: 1-complete all the points in the needs and delivrables section. 2- make sure that the logs are free of errors after completing all the above points. 3-the above two points should be done on our server not your server while you can do all needed tests and investigation at your end and we will be sharing the website database and FTP to the shortlisted bidder. Notes: 1-please dont waste your time in negotiation, we have fixed budget and price (50 USD) which will not be increased, and to be paid in one payment (final payment) after completing all the fixing at our server. 2-all the communications and payments will be done in this website, please dont approach us by over skype, email, phone, etc. Extra notes: