@Anathanatas nPlease check Troubleshooting issues.
After you checked all items in Troubleshooting issues and that did not solve your issue, tell us more:
OS version and build
Installed security tools
like Antivirus, Internet Security etc.
Installed privacy & cleaner tools
Connection
f.ex LAN, WLAN, Mobile, VPN, Proxy etc
Installed extra codecs in OS
Installed third party ad/tracker filter or blocker
Installed latest chipset/GPU driver
from graphics producers website
Vivaldi version
Open page vivaldi://about
Use of Vivaldi ad/tracker blocker
Activated block lists
Info about changed settings in page vivaldi://flags
Installed extensions
Open vivaldi://system
Section extensions
Button Maximise…
Copy the items from list in row extensions
Paste here with </> button
Info about GPUs
Open vivaldi://gpu
Hit button Copy Report to Clipboard
Paste here with </> button
Updated DRM codec
Open vivaldi://components
Scroll down to "Widevine Content Decryption Module"
Post version number here
Try to update Widevine with "Check for Update" button
Info about media usage
Open vivaldi://media-internals
Click on Copy to Clipboard
Paste here with </> button
Visit the following links and tell us which options are selected for the selected flags:
vivaldi://flags/#ignore-gpu-blocklist
vivaldi://flags/#disable-accelerated-video-decode
URL where issue happens and if a player is used on webpage, the setting for video or audio format
Information if a Vivaldi downgrade was done before with the current browser profile