Skipped Attachments (code 3525)

This article applies to Microsoft 365/Google Workspace Backup and relates to Microsoft 365 attachments to Calendars.

Situation

A backup plan finishes with the following warning message:

The attachment is too large to back up and was skipped... (Code: 3525)...

The list of skipped attachments is available in the Backup History.

Cause

The reason for this error is a Microsoft limitation. System.Text.Json sets limits that can't be changed for some values, such as the maximum token size in characters (166 MB) and in base 64 (125 MB). For more information, see JsonConstants in the source code and GitHub issue dotnet/runtime #39953.