Nop 3.90 uses old ASP.NET Core
Open the file global.asax at second instance and add space to the end of the file. Save it.
IIS will recreate the >NET cache.
>Otherwise plugin would not activate on that 2nd server.
Check if the second instance uses own cache.
The first and second instance have to use the same cache (ex. Redis).
>I tried uninstalling on server1 and installing on server2.
Install plugin on one instance and change the file plugins.json in the second instance.
But I think you will not be able to configure two instances of nopcommerce withot issues.
I tried to do it for one my customer several years ago.
Each instance has own nopcommerce task manager and this task manager works on own instance.
You use plugin for lazy-loading of pictures. Disable it and try again.
Send me your store URL (urls for pages with wrong pictures)
Check if your themes or external plugin uses Ajax POST requests for getting pictures.
Disable such plugins and try it again.
This topic is for one customer.
Фумігати - Джин
Тернопіль
https://www.solfum.com.ua/production/fumigat/jin