Sorry if 'Custom toolbar module' is not the right term for the module on the toolbar where you can put in different menu or command buttons, I'm not up with the terminology.
 
I had assigned a button to "Insert Marker", but since the 2nd latest update, nothing happens when I click on it. (I can still insert a marker the old way. And other custom buttons still work.) Anyone else had this issue?

Hi. Have you tried to unbind, and re - bind your "button" in Preferences? It could help.
All the best.

Try simply to unbind your button and save OK. For me it's working.

I think all these 'shortcut x not working' issues are due to having the Synth Rack docked at the upper right. Hide it or dock it somewhere else, and see if the problem goes away.

Thank you Ken and Lapasoa. Forgive me, but I've never unbound a button in my life - wouldn't know how to. I looked under preferences, Ken, but couldn't find the right sub menu for unbinding buttons. Can you specify the path?
 
Thanks also to you Brundlefly, but the problem surfaced upon new update at which point I didn't have the synth rack docked anywhere - in fact I couldn't even find the synth rack in the browser section where it used to be, and had to bring it up manually in the views menu.
 
All my other custom buttons are working: e.g. insert tempo, quantize. It's just the Insert tempo button.

Go to Preferences (P), make sure Advanced (blue) is checked. Scroll down to Customization, Keyboard Shortcuts. Press Locate Key (Red). When dialog box (orange) opens, click on your "button". It should be seen as "bound" to Insert Marker with a black line. Click Unbind, then check both, and click Bind. 

 
The new shortcut for the Synth Rack is Alt + 9.
All the best.
 

Thanks Ken for all the effort you went to explaining this to me and posting a screenshot. I very much appreciate it.
 
I will get some free time in a couple of days and will try out what you have outlined.
 
All the best