Is it anyhow possible? There's some defaulty added which i never use.

This generally happens when Opera crashes. Have you been experiencing crashes?

no

Does Opera automatically disable add-ons? When i go to the add-ons page, some add-ons are sometimes disabled, even i have not done that. I have the latest Chromium.

Personally I've bitten the bullet and stopped using Opera 12 for javascript heavy sites like Facebook and YouTube, it works but is crawlingly slow and prone to all sorts of malfunctions.
I still use 12.18 for 90% of my web surfing, but it has to be accepted that for an increasing number of sites it just doesn't perform any more as it should do.

Excactly. It took a while to get used to Opera Chromium, but it's a pretty good browser. All though it's quite heavy on older systems.

It's probably same as deleting plug-in data. It does clear all possible changes you have made to the flash player via flash settings manager.

When viewing (at least certain) html5 with the latest stable Opera on Windows 8.1, CPU usage is almost 100%. I checked the Opera's task manager and 'Widevine Content Decryption Module' seems to be main reason for that. It's my mom's ASUS K73BE laptop. With my desktop computer WCDM does have hight CPU, when watching those same videos. Is it just that AMD E2-1800 processor is not powerful enough to run those vids? Here's one example: http://www.katsomo.fi/#!/jakso/33005099/penny-dreadful/626279/uusi-helvetti I'm not sure which resolution that vid is.

Not sure if this video is accessible in other countries.

Also, tried toggling HW video decoding and the browser's HW-accelerating on and off, but that din't help. Also reinstalling Opera did not help.

SOLVED! Flag option "Start page WebUI components" needs to be enabled. Interesting enough, it didn't need to be enabled in Opera 37 for those side panel buttons to work.

I uninstalled Opera and did a standalone installation, and those side bar buttons still won't work. The latest Opera Stable and Windows 7.

Clicking the history button on the start page just refreshes the start page. Same happens with the bookmarks button.

Also found, that embedded instagram videos needs 3 clicks before starting to play. And this is with Opera 37.

I've tried here and all started playing at first click.

Yeah, it was due the "disable html5 autoplay" extension. Didn't realise it first, since there's no controls in the video- playback bar etc. And i keep the extension icon hidden.

Also found, that embedded instagram videos needs 3 clicks before starting to play. And this is with Opera 37. Here's an example: http://www.iltalehti.fi/fiidifi/201606010122044_fd.shtml

With Opera 12 those videos won't play at all.