Please also update .net to latest version from microsofts website
What you are saying doesnt add up. An upgrade would not cause a version compatibility issue it will be crucial you provide us the crash log so we can see what the real problem is.
5.3.6 to 5.7.8 is a HUGE jump we can help you with it but it is not surprising that something may be a little off. However it should all be pretty easy fix just let us know what the error log says.
We do not provide the older versions anymore. It would not take very long to install 5.7.8 and tell us the crash log… probably faster than finding 5.3.6 from someone. Crash log will help us solve things a lot faster.
A few things i have seen were related to the database table structure changes that have been made since 5.3.6 perhaps it needed newer .net framework installed. Perhaps it was related to the new time clock tables which may need some fixing since its such a huge jump.
A lot of issues are resolved by making sure you install the latest .net framework
If you already bought API license, you will need version v5.5.0 or up.
If you already bought an API license, Id recommend you to use newer version as it has updates to addEntity gql endpoints, which make it easier to manage custom data for entities too for your reservation platform.
Ok well I think Posflow has helped you in that regard. Best of luck to you… When you do decide to try and upgrade again and run into the crash please paste the crash log here and we can help you with it.