First of all - I reported this error using Opera Bug Report Wizard on May 28th while it was happening in Opera Beta. Bug report got ID: DNAWIZ-29482. I didn't get any reply.

You can reproduce this error only if your mouse has extra backward/forward buttons.

Clicking mouse back button goes back by 2 pages instead of 1, example:
I'm on site A then I navigate to site B and then to C. Pressing backward mouse button gets you to site A instead of site B

When i click and hold back button on my mouse then opera goes back by only one site, then I release that button and it goes back one more page. Same thing happens with forward button. I checked that on 2 different computers running Linux Mint with 2 different mouses. Button numbers in xinput test are the same for both computers so I assume that it's error that could possibly affect more users than just me. Strangely enough this behavior does not occur on Windows.

I checked which buttons are clicked exactly on my mouse by terminal command "xinput test <device_id>", it seems that someone changed opera's behavior by triggering forward/backward navigation on both key press and release:
button press 8
button release 8
button press 9
button release 9

@tina
Bug has been resolved in 54.0.2952.54
Thank you!

@s390g2 Thanks for old stable ftp link!

@bombacha I didn't have that problem until version 54. On 53 every video worked without problems. Even if this is some kind of licensing problem then would it be so hard to add popup on fresh opera start and ask user if he/she wants to install additional codecs? It wouldn't. Look at proprietary intel/nvidia drivers on ubuntu - all you have to do is click button to install them.

edit:
Another funny thing is that I can't seem to find .deb packages for older opera releases. I looked for "Opera 53 deb" through first 2 pages of google results - every link that I found downloads the newest release.

First of all - I reported this error using Opera Bug Report Wizard on May 28th while it was happening in Opera Beta. Bug report got ID: DNAWIZ-29482. I didn't get any reply.

You can reproduce this error only if your mouse has extra backward/forward buttons.

Clicking mouse back button goes back by 2 pages instead of 1, example:
I'm on site A then I navigate to site B and then to C. Pressing backward mouse button gets you to site A instead of site B

When i click and hold back button on my mouse then opera goes back by only one site, then I release that button and it goes back one more page. Same thing happens with forward button. I checked that on 2 different computers running Linux Mint with 2 different mouses. Button numbers in xinput test are the same for both computers so I assume that it's error that could possibly affect more users than just me. Strangely enough this behavior does not occur on Windows.

I checked which buttons are clicked exactly on my mouse by terminal command "xinput test <device_id>", it seems that someone changed opera's behavior by triggering forward/backward navigation on both key press and release:
button press 8
button release 8
button press 9
button release 9

@bombacha It's not a solution at all, Opera development quality is worsening with every version. I'm using Opera Beta and this issue was introduced with Opera Beta 54 somewhere around 26-27th of May. After a month of "testing" same bug goes straight to Stable version...