@burnout426 Eventually, I found out, that the problem is not associated with Opera but as you said, it only turned out when I used it. It's a problem with DNS resolving, indeed, but it was happening with Firefox or Brave as well.

My ISP advised me to make a complete reset, so I'll hope that the issues are gone now. Thank you for you help, nevertheless. You're advice is very welcome.

@burnout426 Eventually, I found out, that the problem is not associated with Opera but as you said, it only turned out when I used it. It's a problem with DNS resolving, indeed, but it was happening with Firefox or Brave as well.

My ISP advised me to make a complete reset, so I'll hope that the issues are gone now. Thank you for you help, nevertheless. You're advice is very welcome.

I don't get a 128 version, not the previous nor the update. When I download the offical installer for Mac it's still Opera 127 on Chromium 143, why this? I tried it twice today.

@burnout426 First of all thank you for your help.

Actually I changed the settings in Opera both desktop and Android alike to: DNS over HTTPS via Cloudfare right after installation. I just thought that it was right that, that caused the trouble. But disabling it, didn't help either.

I haven't made a test on another Chromium browser yet. But I remember a time when I had another ISP with some other hardware (router and a Linux laptop in this case) I saw some similar behavior when using Vivaldi. I saw some other people posting similar issues on their forum then.

Actually, my impression is that it doesn't generally happen with Opera but only on specific versions and it seems to me, that it's version 128 on desktop or the respective version on Android that might be causing the trouble.

Yet, I threw it at Gemini and apart from the VPN thing it gave me this answer:

The FritzBox Reaction: Some FritzBox firmware versions have a security feature called "DNS Rebind Protection" or automated DoS (Denial of Service) protection. If Opera attempts to open hundreds of simultaneous connections to a single proxy IP to "speed up" loading, the FritzBox might flag this as suspicious activity and temporarily throttle or drop the connection to protect the network.

2. Synchronization Stress
You mentioned a "fresh install" on Mac, iPad, and Android all at once.
When you sign into an Opera account on three devices simultaneously, they immediately begin a massive sync of bookmarks, history, open tabs, and extensions.
If your FritzBox 6660 is running older firmware, the sudden burst of "Stateful Packet Inspection" (checking every single one of those sync packets) can occasionally cause the router's NAT table to overflow, leading to a hang that requires a reboot.

3. BitTorrent / Flow Integration
Opera has built-in features like Opera Flow and, in some versions, native support for certain file-sharing protocols.
Unlike Firefox, which is very "standard" in its traffic patterns, Opera’s sidebar features and "My Flow" use a specific type of peer-to-peer (P2P) communication to keep devices in sync.
Cable routers (like the 6660) are notoriously sensitive to high numbers of UDP connections typically used by P2P protocols. If the router's processor spikes to 100% trying to manage these, it will keep the Wi-Fi alive but stop routing internet traffic.

I haven't changed the settings in the FritzBox itself yet as it's working for Firefox now. But I may do that in a while.

This is really annoying but it happened again yesterday. After having installed and used Firefox for a day without any issues I tried the same with Opera yesterday. Worked for half a day, actually, but in the evening when I was going to stream from it, my connection went downhill.
I restarted the router for a - I don't know how many the last few days - further time and deinstalled Opera from my devices turning to Firefox and it simply worked.

I'm giving Opera another try. It's too much I miss from it. Speed dials, the handy split screen feature, the sidebar to say the least.
But I keep an eye on my internet connection.

It's possibly crazy and seems to be weird, but the last days it seems to me that installing and using Opera on my devices seems to trigger issues with my router, a FritzBox Cable 6660.
I asked AI for a reason and it gave me back: Normally no issue possible like this because of one browser but in some cases it might be.
I installed Opera on my MacBook and on my Androids the other day and only after a while my internet was disturbed and caused me to restart my router.
I've deinstalled Opera and used Firefox for the time being and there haven't been any hiccups since then.

I wouldn't have thought this was a fault stemming from Opera if I hadn't something like this with earlier versions. It seems to dependent on Opera's way to resolve DNS probably which triggers something in the router to close the connection as if it were attacked as far as I understood.
I'm not sure if this comes from the desktop or the mobile version or even either. But if you asked me, I'd think it's mainly the Android one.

Unfortunately this led to me to turn to Firefox for now. I just wanted to give note to this maybe very unique behavior.

Habe gerade durch Zufall entdeckt, dass der Fehler schon gemeldet wurde: https://forums.opera.com/topic/87970/opera-127-0-5778-76-stable-update/23

@roflthecow Ok, willst du das mal im englischen Teil des Forums posten? Ich fürchte, hier liest das niemand von Opera.
Du kannst auf jeden Fall @Opera-QA-Team taggen.
Meiner Erfahrung nach bleiben solche Probleme oft über mehrere Versionen ungelöst, bis sie dann - schließlich - von selbst wieder verschwinden.

@roflthecow Diese Funktion kannte ich gar nicht, aber ich kann das tatsächlich bestätigen. (Ich nutze die Lesezeichenleiste in Opera sonst gar nicht), Ich habe mir einen Ordner aus der Speed Dial in die Lesezeichenleiste gezogen und die einzelnen Seiten öffneten sich wild durcheinander. Zur Kontrolle habe ich das auch bei Firefox gemacht und, siehe da, alle Lesezeichen öffneten sich in der Reihenfolge des Ordners.
Meine erste Frage wäre dann: Hat sich das Verhalten in Opera geändert, z.B. durch ein Update? Oder war das schon immer so?

@nohookneeded Ich kann deine Anregung gut verstehen. Allerdings würde ich diesen Wunsch auf Englisch in dem entsprechenden internationalen Teil des Forums posten:
https://forums.opera.com/category/49/opera-browser
Erfahrungsgemäß liest das hier niemand von Opera. Und es gibt auch keinen deutschsprachigen Moderatoren hier.