Excel not opening after macOS Tahoe 26.0.1 update

“Microsoft Office up to date, works fine, except for old (2020) Excel file, which it says cannot validate developer. Problem since upgrading to OS Tahoe 26.1.”

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MS Office Seen as Unidentified Developer:

So, where did you purchase this? It if is NOT from a retailer nor from Microsoft, then it may be an invalid license key —officially meaning it is fraudulent and malware. Developers of software often create their own kind of files, making them be seen as scams. So, if purchased elsewhere, you should:

1. back up your Mac using Time Machine Backup
2. uninstall MS Office
3. purchase MS Office from your local Retailer or directly from Microsoft
4. Install that version
5. create a backup again

System Integrity Protection:

Back in the day of High Sierra, this whole “unauthorized developer” became a big ordeal, and the only way around that was to use the following link, or use a command in Terminal. This may do the job: Open a Mac App from an Unknown Developer - Apple Support

Scan your Mac for Malware: 

Use MalwareBytes for Mac --- This is software that searches for malware/adware. So, scan with it and then remove what is found. Once removed, uninstall MalwareBytes for Mac. Then restart the Mac.

Downloads:

1. Malwarebytes Anti-Malware for Mac
2. Malwarebytes Uninstaller

Note:

Microsoft at times can be little picky and demand that your only option is to call them. That typically occurs when you need assistance with getting into your account. But, being not the case, they ought to run you right through it. I’d say it’s no more than a glitch. So, ask Microsoft for a new key while in the chat.

Go here: Contact Us - Microsoft Support

"Excel not opening after macOS Tahoe 26.0.1 update: Lost Excel from Microsoft 365 all the other apps are fine. Believe this is the rust of OS update to Tahoe 26.0.1 Can't open Excel"

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Contact Microsoft:

Ask Microsoft for a New License. If provided, they'd have you Uninstall the app > restart the Mac > Install the app with the new key. Go here: Contact Us - Microsoft Support

No issues here with latest Excel, and before testing I upgraded to Tahoe 26.1:

[M4 Pro Macbook Pro 14[

Did you try restering the computer?

Have you installed any useless utilities like anti-virus or so-called "cleaning" apps?

Microsoft Office up to date, works fine, except for old (2020) Excel file, which it says cannot validate developer. Problem since upgrading to OS Tahoe 26.1.

Same here. All other 365 office apps are working well, but Excel not opening

“Same here. All other 365 office apps are working well, but Excel not opening”

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See my above reply.

Start reading the posts above yours for your answer. Well, actually you didn’t ask a question did you. And I doubt we’ll hear back from you either will we.

But for what it’s worth Excel is not an Apple product so you’ll need to contact Microsoft Support for help. Good luck.

“I have lost excel from Microsoft 365 update following installation to Macos 26 Tahoe”

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Ask Microsoft for a New License Key:

MS Excel is part of the MS Office Suite. So, that means you need a new license for MS Office —you cannot get Excel as its own software. When my Mac computer got damaged not so long ago due to a ceiling leak, I informed MS of that incident. Kindly enough, the MS rep told me “You own that license —here is a new key for you.”

Go here: Contact Us - Microsoft Support

My Excell is working but it starts to "loading excel startup folders"and then a loop series of opening pdf, docx and other files and can't stop it. have to force quit it, otherwise it keep on load docs and corrupted files and no chance to use excel.

"My Excell is working but it starts to "loading excel startup folders"and then a loop series of opening pdf, docx and other files and can't stop it. have to force quit it, otherwise it keep on load docs and corrupted files and no chance to use excel."

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Reinstall Office:

Sounds like you have a corrupt install. So, contact Microsoft, requesting a new key. You'll likely have to provide a proof of purchase; you'll find the download file in the subscriptions site of your Microsoft Account.

1. Log In Here: account.microsoft.com/billing
2. Select: "Past 7 Years" for "Ordered Within"

Screenshot from Microsoft's Site:

"Would agree if Excel hasn’t been working properly for 4 years within MSFT 365 subscription right before upgrade to Tahoe 26.0.1"

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Excel Issues:

Problems with MS Excel seems to be getting a good reputation on these forums. So, I'd say it's a bug within Apple Update --Apple and Microsoft aren't too fond of each other, but they know customers are loyal to MS Office. So, without reluctancy, try the reinstall. If not, then get a new license.

"Excel not opening after macOS Tahoe 26.0.1 update: Lost Excel from Microsoft 365 all the other apps are fine. Believe this is the rust of OS update to Tahoe 26.0.1"

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To the OP and ALL those Affected:

This seems to be catching on. So,...

Report this to Apple:

Do your Part: Report this to Apple. Apple may not get back to you directly, but the more feedback Apple receives on this, the more they will know what is going on, and what needs to be fixed in upcoming updates.

1. Go Here: Feedback - macOS - Apple
2. Select: "Bug Report" for the "Feedback Type"
3. Comment: that this all began after installing an update, and how you have troubleshot it thus far. Include the URL to this thread.
4. Proceed from there as necessary

I have lost excel from Microsoft 365 update following installation to Macos 26 Tahoe

Would agree if Excel hasn’t been working properly for 4 years within MSFT 365 subscription right before upgrade to Tahoe 26.0.1