I am glad that at least we resolved one bug.
About the _PS_MODE_DEV_, could you please do the following:
- Disable debug mode.
- Enter BO to get an error.
- Locate PHP error log.
- Copy and paste the error onto here.
I think that your problem is a quick fix, we just need to find the problem. Meanwhile, I will look into your deprecation warning.
EDIT: You can easily remove sensio deprecation message.
- Go to ROOT_FOLDER/app/config/config.yml
- Place the following code after Doctrine Configuration (around line ~105)
# Sensio Configuration sensio_framework_extra: router: annotations: false
NOTE: PrestaShop 1.7.6.x will most likely solve these issues so before upgrading make a backup.