If you don’t need the UiPath.Web.Activities in the Project loading the aside from within the Library Activities, don’t include it in the calling Project. When you add the Library Package it will download any required Packages, so having both of these (despite your setup that SHOULD be correct) can cause conflicts. I would avoid overlapping Packages if you can.
The feed is located in the “Manage Packages” Window under the “Settings” tab. You will see the default feeds on the top area, make sure “Official” is enabled, but seeing as you’ve added the UiPath.Web.Activities Package, it should be enabled already.
The next thing I would try, is in your Library, I would try and make sure the default Packages (UiPath.System.Activities, UiPath.Automation.Activities, etc) all match the version from your calling Project. I’ve had issues where despite setting them to “Lowest Applicable” they still encountered errors that prevent the Library from being accessible within the calling Project. I had to just remake the Library, I just pasted all the .xaml files in the new Library and didn’t play with the Package versions and it fixed itself.
I hope some of this blurb can help you figure out why you’re getting conflict errors!