I had the same problem as @szczepanson it started working for me after following steps:

• changing the display language to english
• restarting Opera
• sigining out and signing in again in the synchronization menu

Without following steps the opera://activity were working fine but he icon on the task bar was missing and not available for selection.

I am using Opera in version is 60.0.3255.165

@szczepanson yes, for me steps I have described above fixed the problem permanently.

I had the same problem as @szczepanson it started working for me after following steps:

• changing the display language to english
• restarting Opera
• sigining out and signing in again in the synchronization menu

Without following steps the opera://activity were working fine but he icon on the task bar was missing and not available for selection.

I am using Opera in version is 60.0.3255.165