Not sure if I'm late to the party, but I get the same loading errors in Sketchup 2020 (pro). I'll put the error codes here down below, maybe someone knows what's up:

Error Loading File C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/MSPhysics/main_entry.rb
Error: #<IOError: The required, staged c extension file, "C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/MSPhysics/libraries/stage/win64/2.5/msp_lib.so", is missing!>
C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/ams_Lib/extension_manager.rb:204:in block in require_all' C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/ams_Lib/extension_manager.rb:201:in each'
C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/ams_Lib/extension_manager.rb:201:in require_all' C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/MSPhysics/main.rb:70:in <top (required)>'
C:/Program Files/SketchUp/SketchUp 2020/Tools/RubyStdLib/rubygems/core_ext/kernel_require.rb:59:in require' C:/Program Files/SketchUp/SketchUp 2020/Tools/RubyStdLib/rubygems/core_ext/kernel_require.rb:59:in require'
C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/MSPhysics/main_entry.rb:15:in <top (required)>' C:/Program Files/SketchUp/SketchUp 2020/Tools/extensions.rb:197:in require'
C:/Program Files/SketchUp/SketchUp 2020/Tools/extensions.rb:197:in load' C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/MSPhysics.rb:21:in register_extension'
C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/MSPhysics.rb:21:in <module:MSPhysics>' C:/Users/ThisPC/AppData/Roaming/SketchUp/SketchUp 2020/SketchUp/Plugins/MSPhysics.rb:5:in <top (required)>'
Error Loading File s4u_scaletool/s4u_scaletool_menu.rb
Could not find included file 's4u_scaletool/s4u_scaletool_menu.rb'