Outlook Classic issue: Shows suddenly an "Add a service" popup and ask for credentials (June 2025)

[German]A blog reader told me about a strange problem in Microsoft Outlook Classic. Since June 13, 2025, Outlook shows a few minutes after start a dialog box "Add service" and disables the internet connection. It affects only single clients in two enterprise environments. Perhaps a reader has an idea of what is behind this.

The environment for Outlook

The blog reader, who does not wish to be named, wrote in his email dated June 25, 2025 that he had come across a very glaring error in his environment, which can be observed at two locations of a customer. According to his statement, the IT systems at both locations are independent, i.e. two overall Active Directory (AD) structures are managed.

Two Microsoft Exchange servers have been running there for years, whereby an Exchange Server 2019 has always been used at location A. Site B first used an Exchange Server 2016 and now an Exchange Server 2019. However, the reader wrote that they in IT department do not assume that it is due to the mail servers. The customer's company uses Outlook Classic clients from Microsoft Office 2016, 2019 and 2024.

Credential error in Outlook 2024 since June 13, 2025

Starting on June 13, 2025, the following error occurred simultaneously "with both Office 2024 versions" (probably meaning with the Outlook 2024 clients at both locations).

• Microsoft Outlook 2024 can be started normally and emails can also be written and successfully send.
• After approx. 1-2 minutes, an popup "Add a service" is showing and asks for credentials of a service (see the screenshot below).

The pop-up with the title Add a service informs the user that no work or school account was found with this e-mail. According to the reader, the user principal name (UPN) of the user is automatically entered as the user. No matter what you enter, it is not successful.

• If the user closes the window, Microsoft Outlook is disconnected from Exchange.
• If the user opens Outlook again, everything works again until the window with the title Add a service appears again.

The reader and his colleagues then took the usual steps to resolve the problem. According to the reader, the following was tried without success:

• Login vault deleted
• New Outlook profile
• Old device deleted, which was stored for the MS Home and Business account
• Latest Office updates for 2024 installed (10, June 2024)

According to the reader, the following registry values were already set in advance, so this is also not a solution to the problem:

Even deactivating the data protection setting in Outlook Classic did not solve the problem outlined above. Subsequently, reinstalling the Office 2024 versions was a temporary solution. This is because the approach worked with one Outlook client, while the problem continues to occur with other Outlook clients.

Uninstalling the CoPilot app and trying to adjust various Outlook settings, switching off cache mode or not loading shared mailboxes did not provide a solution either.

Outlook 2016 and 2019 suddenly also affected

The reader wrote in his initial email that it suddenly became very exciting. While initially only Outlook 2024 was affected, the problem occurred again on 25.06.2025 with two clients (1 client per location), but now Microsoft Outlook 2016 and Microsoft Outlook 2019.

All remaining clients (approx. 30 to 40) continue to function normally at the current time. The reinstallation without the current patch status has solved the problem with MS Outlook 2016 for the time being.

Microsoft support not very helpful so far

The Office 2019 computer has now been left as it was when the error occurred. IT then opened a critical support case, including remote support, with Microsoft. The only help from support so far has been the advice to set the two registry values mentioned above to 0, restart the clients once and then see what happens.

The issue isn't an isolated case

In the meantime, the blog reader has carried out some internet searches and has come across hits in various forms. Here are some articles on the same problem:

• Einen Dienst hinzufügen Outlook Meldung geht nicht weg (MS-Answers)
• Mein Outlook möchte "Einen Dienst hinzufügen" und hört damit nicht auf (MCSE-Board)
• Add a Service Credential pop-up in Outlook Critical issue (MS-Answers)
• Constant Microsoft "Add a Service" account pop-up that will not stopp (MS-Answers)

After publishing the German edition of this blog post, other administrators confirmed that they are facing the same issue on some clients in its enterprise environment.

Browsing the articles linked above, shows a number of suggestions given in the forum post, what can be done to fix the issue. These suggestions reflect what the reader has outlined above and tried without success. There is no explanation or real solution.

One user writes in this thread that they are using Exchange 2019 and have upgraded to Outlook 2024 from Outlook 2016. There, too, users received the pop-up in Outlook asking for their Microsoft 365 login information. IT then opened a support case and received a registry fix In the current scenario, the registry entries were missing (see here for the registry keys). These were added and the clients were restarted. The problem had disappeared. However, it did not help in the blog reader's case above.

It gets even crazier: Fiddler stops popup

In a follow-up email, in which I also received the go-ahead to publish the case, the reader mentioned that they had come across a totally crazy issue. The customer had tried to recreate the problem on an affected Outlook 2016 client and had the communication recorded using Fiddler. When Fiddler is running, the pop-up does not appear […].

A German blog reader explained, that Fiddler creates a proxy, that probably influence the behavior of Outlook connections. But so far, there is no real clue what's going on with this issue and what's triggering the "Add a service" popup asking for credentials.

At this point, the reader ran out of ideas (original quote: "We ourselves have at least 80 Exchange servers in our support, it feels like the customers have thousands of clients with Office", but the problems don't occur there). The reader then pointed out that the Office in question was an MS Office Home and Business SKU. He concluded with the sentence: "I hope to be able to find solutions via their reach, because it seems that Microsoft itself is overwhelmed by the problem."