Panel icons show as white squares
My panel icons used to show perfectly fine in the panel.
(I first had to whitelist in dconf-editor > desktop > unity > panel > systray-whitelist > ['all'].)
Just recently, however, icons have been showing as just a white square.
This does not affect all icons. The built-in ones show normally; Skype shows normally; Truecrypt shows normally but on a white square background; and others, such as Libre Office Quickstarter, SpiderOak and yad all show as just a white square.
See the screen-shot; on the left are three icons, viz. Truecrypt, Spideroak and Libre Office.
Any idea how to fix this?
Re: Panel icons show as white squares
I restarted my machine this morning, and the problem has quite mysteriously been solved. I wish I knew why.