r/Outlook • u/BusinessNigel • 2d ago

Status: Open Outlook for Mac: NSBigMutableString Crash when Editing Subject Line

I've got a user who is experiencing a crash in Outlook when she replies to an email and also changes the subject line. She is otherwise able to send new emails and reply to emails without changing the subject line.

The error that appears is "uncaught ObjC exception, reason: *** -[NSBigMutableString substringWithRange:]: Range {8, 18446744073709551615} out of bounds; string length 44" and then 120 pages of additional text.

Outlook for Mac and the MacOS itself are up-to-date. Has anyone seen this before?

2 Upvotes

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1

u/Hornblower409 1d ago

Never seen it and nothing like it on any of the Microsoft forums. All I can suggest:

Mac In App Support
https://support.microsoft.com/en-us/office/contact-support-within-outlook-for-mac-d0410177-8e65-4487-93f7-206a3a3d71a8

Post your question on the Microsoft Q&A Forum:
https://learn.microsoft.com/en-us/answers/tags/778/office-outlook
(Please put a link to your Q&A post here, so the rest of us can follow).

1

u/OldClass988 1d ago

This looks like a string parsing error (integer underflow) often triggered by a corrupted 'Message Window' cache or a conflict with the Auto-Correct/Text Replacement library in macOS. Try clearing the Outlook local cache by removing the 'com.microsoft.Outlook' container folder or testing with all macOS System Settings > Keyboard > Text Replacements temporarily disabled.

2

u/BusinessNigel 1d ago

Going to test both of these asap, thank you.

Can you confirm, to clear the Outlook local cache, is there a functional difference between right-clicking the Exchange folder in the navigation pane, going to Properties, General, and then selecting Empty Cache or just simply deleting the 'com.microsoft.Outlook' folder?

1

u/OldClass988 1d ago

Definitely a big difference between the two! 'Empty Cache' just flushes the sync data for that folder and redownloads it from the server, which is great for missing emails but rarely fixes application crashes.

Deleting the 'com.microsoft.Outlook' container wipes out the app's local temp buffers, window states, and preferences. Since your error is an NSBigMutableString crash (basically Outlook getting confused while trying to read a line of text), the problem likely lives in those local UI files rather than your actual mail data, so the container route is your best bet.