According to the project's creator, Notepad++ Plugin Manager's update URL is hosted on nppxml.bruderste.in
Hosting was moved from *.sourceforge.net
to nppxml.bruderste.in
in October 2016 due to a controversy where Sourceforge began bundling adware with installers for abandoned projects.
From the sponsor page for NP++ Plugin Manager
Since 2010, Plugin Manager has been hosted on Sourceforge.net. They've
been a great host for us, and generously supported the large amount of
traffic that the plugin manager generates. To put some numbers on the
traffic, whilst the plugin list is only 50KB, and is normally only
download when it changes, each change generates around 500GB of
traffic, with a steady 200GB of traffic each month.
In 2015 there was some controversy surrounding SourceForge bundling
adware with abandoned projects. This has since been cleared up, and we
applaud SourceForge for taking the right path in this regard. However,
this sparked some discussion and thinking around what we could do to
transfer the hosting of the list off SourceForge and onto another
provider, and what benefits and disadvantages that would bring. There
are things we aren't able to do easily with the sourceforge hosting
system, and relying on the goodwill of a website is not good in the
long run for the project. Using a more traditional hosting platform
would allow us to address some of the current issues we have around
automating the list updates, and serving a dynamic "bleeding edge"
list.
So, I'm very grateful to Nexinto that they are sponsoring the hosting and bandwidth requirements for Notepad++ Plugin Manager, and hope you can forgive the small logo and link in the plugin which means we are reliably hosted on VMs that we control, and hence can extend what we do, bringing automatic fortnightly releases of new plugins to the plugin manager.
For reference, the old url was https://downloads.sourceforge.net/project/npppluginmgr/xml/plugins.zip