As of this morning I am no longer able to upload a custom icon for a project.
After loading, the preview shows the broken thumbnail icon.
Does anyone else experience this problem?
Seems to be working fine for me, @Herve_Buisset.
Perhaps try these troubleshooting steps, (particularly try the incognito/private tab) and if you still have an issue, then please create a support ticket, by following the instructions here: How to contact our Support Team 
Thanks Richard. Sometimes one forgets the basics … clearing cache and relaunching chrome did the trick.
Indeed 
Glad that worked…!
Our team has had this issue all day with custom icons that have been working until today. We’re seeing the same issue across different OS/browsers/etc. I’m on a PC using Chrome and went through the troubleshooting options (including clearing cache etc. and incognito mode) to no avail. I’ll create a support ticket if it isn’t back up and running by tomorrow; just adding this here in case this becomes an issue for other folks.
It would be great to have a custom icon library so we don’t have to upload them for every project. Someday!
Hi @meg.meg.meg , being on an Enterprise plan, support should get back to you soon.
However, regarding having a custom icon library (which I agree, would be great!) you can vote for that, here:
Update: This function is now working again for my entire team without further action on our part.