Other than previous versions of Opera (I have 35.0.2066.68) the Flash plugin does not show up in the Opera-plugin-list and therefor cannot be enabled. Websites detect the flash plugin as missing and request installing. But Flash is installed on the PC and is used by Internet Explorer without problems.
Can this issue have something to do with flashblock-plugin? However, previous versions of Opera had no problems with both flash plugin and flashblock.