Opera blacklisted the version of Tampermonkey that is currently offered on the Chrome Web Store as it is being installed by Windows malware. This prevents the extension from working in the Opera browser.
Opera users who installed Tampermonkey 4.7.54 from the Chrome Web Store found out today that the browser has blacklisted the extension. When they open the browser's extensions list, they are greeted with a red message stating that the extension was blocked because Opera "identified this extension as malicious and have blacklisted it. This means it can no longer cause any damage to your machine. You can leave it as is, or remove it."
On the other hand, users who installed Tampermonkey from the Opera store would not have any problems, because it is the older 4.2.5291 version.
When Tampermonkey developer Jan Biniok heard about the issue and contacted Opera, he was told that the browser company has detected malware installing the version of Tampermonkey found on the Chrome Web Store with extension ID dhdgffkkebhmkfjojejmpbldmpobfkfo, using alternative distribution options. What this means is that the extension was being installed manually via the Registry or a JSON file.
Tampermonkey dev's response
The message then goes on to say that the installed extension is being "used as a vehicle to circumvent our acceptance criteria for browser extensions."