Not sure what happened, but loading PNG stopped working for me. When I click "Load" icon or choose "Import frames from files", then select PNG images - nothing happens. JPG images work fine.
This seems to affect Chrome only.
I see this error in the console log:
projectResources.js:1775 TypeError: Cannot read property 'sg' of undefined
at new e.SHc (projectResources.js:2027)
at Function.d.l (main.js:291)
at Function.h.Wb.vfb (projectResources.js:1874)
at k.RHc.a4 (projectResources.js:1775)
SOLVED! "Enable experimental features" setting was causing this.