PayPlans 4.0 Stable

I have just got a new membership, and having error installing PayPlans version 3.5.5.
Also, the installation prevented me use my previews version 3.5.1(that disappear from the menu and clicking on its links on the frontend trows a 404 error), what is DAMAGING my business.

I tried first to update via Joomla updated, but it seams to search for the old Readybytes website.
I tried following instructions on the dashboard page of your site:
Installing 3.5.5 package gives me "Error while processing task: download".
Installing 3.6.3 package directly give me "Some fatal error occured!" (images attached)

Have cleared cache several times between one installation and another.
Have checked chmode of files/directories and they are ok.

Please need help and instructions what to do ASAP

Thank you
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