Masks in Lightroom Classic suddenly stoped working - from one edit to another

Hello, 

While editing some photos from a wedding I shot I suddenly wasn't able to click on the mask tool. Also, other controls like the histogram can not be slid (to raise lights or darks for example). 

I'm no pro wahtsoever so my idea was restarting the program - It didn't help. 

This is happening on a brand new iMac and a recently downloaded license of Lightroom. 

Thanks in advance.