[M365] Invalid MIME content-length header encountered on read

[admd]

Hello all,

I got a 2021 error on a job targeting OneDrive. Our Veeam is on 8.2 last version. Nothing unusual.
Case is 07887382. I'm about to send the logs for a third time with no clear explanation.

[20.11.2025 00:04:33.293] 107 (1) [Warning] Warning: Failed to backup item version: /_vti_history/7168/Documents/XXX.pst, version: 14.0, Invalid MIME content-length header encountered on read.
[20.11.2025 00:04:33.293] 107 (1) [Warning] Warning: Invalid MIME content-length header encountered on read.
[20.11.2025 00:04:33.293] 107 (1) [Warning] Type: System.FormatException
[20.11.2025 00:04:33.293] 107 (1) [Warning] Stack:
[20.11.2025 00:04:33.293] 107 (1) [Warning] at Microsoft.SharePoint.Client.Mime.MimeReader.ReadHeaders(Int32 maxBuffer, Int32& remaining)

Thanks !

[track1044]

I am seeing this on a few customers as well. I have created a support case awaiting response. Case #07896869

[Polina]

Hi,

This is something new to me. Please continue working with our support to get it resolved faster.

[agrob]

Good Day

We have the same issue. Just opened a case for it 07900144

[JTT]

Same issue with multiple tentants backups.

[Acknowledge]

We've also had a few of these pop up in the last 2 weeks across multiple tenants, started on the 13th.
Veeam asked us to remove the version that was giving the issue.
So far it has been the case that there was a newer version of the file and we could ask the customer to delete the version before that. This stopped the warning.

#07886035

[atha]

Same is happening for us too - we have opened a case but currently no actual tracking from the Veeam support side.
Veeam has been updated to the latest version.
First occurence of the issue November 11th. Year and a half before we never had this.

[jfrobs]

Same problem for one object...

, version: 1.0, Invalid MIME content-length header encountered on read.

[Polina]

RND is researching the issue; the root cause is not yet confirmed. Please keep working with out support - your cooperation helps us to resolve issues faster.

[dkuhl]

seeing the same thing. #03571880

[jfrobs]

For my part, the last ticket with Veeam took four months to resolve, so I'm going to let the others go to war this time and wait patiently in my bunker...

[Polina]

Hi jfrobs,

From a bunker and without an opened support case you may receive the fix way after the others

[jfrobs]

Ahah, I'll keep an eye on the forum, and I'll open a ticket when my other colleagues let me know that there's a fix

[Dagr]

Can confirm I see the same issues
Case #07908235

[mjr.epicfail]

Sounds like Microsoft did a change again....

[Dagr]

@mjr.epicfail
It might be Microsoft yet again...

As I understand the issue it basically means that item data at source is corrupted (I might be wrong here). And MIME data is basically meta data -so it sounds like the MS back-end might be the issue.

Funnily enough, VB365 Version 5b with build number 5.0.1.179 also had a fix for "Invalid MIME content-length header encountered on read"

[jpast]

Same here, Case #07915140

[xlucas]

Here's the answer I got from Veeam support (looks like it came from an internal memo):

The warning 'Invalid MIME content-length header encountered on read' is caused by an inconsistency in the metadata provided by the SharePoint/OneDrive API for certain historical file versions. This is not a Veeam-specific bug, but a limitation in the Microsoft API response for those items. There is currently no definitive fix from Veeam or Microsoft as the issue is under investigation by Veeam R&D.

While the steps below can be helpful, avoid making any changes in the customer OneDrive/SharePoint infrastructure until further notice.
1) Open the site or OneDrive location where the file is stored.
2) Navigate to the folder containing the item.
3) Hover over the file and click on the three dots (···) or Right-click the file.
4) Select Version history.
5) In the version history window, locate the specific version mentioned in the warning (example: version 1.0, 6.0, etc.).
6) Click the dropdown arrow next to that version and choose Download.
If you are unable to manually download those file versions, Veeam will also be unable to backup them because the problem is on the SharePoint side. In that case, it is safe to delete the specific versions that are no longer needed.