Keep in mind that there are straight quotes and single quotes and there are curly quotes and single quotes. One should always work fine no matter what version anyone is using and the other may not display on some setups.
'test' "test" vs. ‘test’ “test”
Note that all quotes and single quotes regardless of direction were done with the '/" key and not the `/~ key.
You can switch between them in mIRC using Ctrl-Shift-' . That may be how things changed for you without knowing that you did anything different and why it was only affecting quotes/single quotes.
As for using 5.91, that is over a decade old. There are MANY changes and fixes including security fixes since then. Using a version that old is just asking for trouble. If they won't upgrade, you shouldn't feel like you have to do anything special just so they can see what you type. It's there fault if they refuse to update.
