Problems with v.2.15 & v1.10 with WP 4.0.1

After I upgraded to WP 4.0.1 successfully. I then upgraded my calendar from v1.10 to v2.1.8 and all the pages with the calendar were mostly blank, just the header was displaying. I also could not get to the backend of WP (/wp-admin) it was all blank. I disabled the plugin by renaming the folder and I was able access WP (frontend and backend) just without the calendar display. So I tried reverting back to my earlier version of the calendar plugin (v1.10) and received completely blank screens for the frontend and backend of the website. So then I manually updated up to v2.1.5 and was able to get the frontend functioning properly with the calendar displaying properly, BUT now I still can’t access the backend. I have to disable the plugin to get to the /wp-admin pages. Please advise. Thank you.