Each plan (package) is limited by number of threads (simultaneous connections).
If you have package with 40 threads and your software uses 41 threads you will see errors and proxies will not work. The same apply if you use proxies on two servers – total number of threads can’t be bigger than what is maximum number of threads in your package.
Some softwares use browser to perform automated actions.
With such software tools you need to lower threads inside software and to upgrade to some of bigger packages.