Error while importing backups in newer Manager versions

I have a Dell Vostro 2520 Laptop with Ubuntu 16.04 OS. Command lscpu shows Architecture: i686 and CPU op-modes: 32 bit, 64 bit. I used to run Manager 16.10.93 version and I was happy with the perfomance. I was using a similar version on other Windows laptops and there were no compatibility issues when I used to import the files across different such machines

Recently I upgraded the version on my Windows laptab to Manager 17.7.55 and generated a backup.
However, when I try to import the backup to another Windows machine that has the latest Manager 17.7.94, it reads ‘Invalid file format’. I had presumed that the backups are upward compatible.

I also noticed that the size of backup file shot up from 180 kB to 848 kB when I used the Manager 17.7.55 instead of the Manager 16.10.93 version.

Again, I tried to upgrade the Ubuntu machine with the latest version of Manager, namely 17.7.94.
Installation gives the following errors.
dpkg: error processing archive Manager.deb (-- install):
package architecture (amd64) does not match system (i386)
Errors were encountered while processing : Manager.deb
I presume that CPU op-modes: 32 bit, 64 bit, would accept amd64 architecture.

Which is the last Manager version that I could download and use on an i386 machine with Ubuntu 16.04?

Only the current version of Manager can be downloaded. There is no public archive of older versions. Search the forum and you will find discussions about 32- versus 64-bit architectures.