All, fyi,
Recently did a clean install of r525 on RPI3, latest Jessie release, Mono 3.2.
I started to get a lot of runtime errors in various programs, all seemed to be "Collection was modified; enumeration operation may not execute."
I did an upgrade of mono from 3.2 to 5.x and all seems to be running fine again (fingers crossed)!
Btw, instructions for upgrading mono below:
# Update Mono
#
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
echo "deb
http://download.mono-project.com/repo/debian raspbianjessie main" | sudo tee /etc/apt/sources.list.d/mono-official.list
sudo apt-get update
sudo apt-get install mono-devel
