Sharegate displays the following error:
The item contains an illegal character. This often happens when there is a vertical tab in the title or description of the item.
Any error code ending with: -269
Sharegate often receives messages from SharePoint in a certain format which has some characters that are illegal and that cannot be used, but SharePoint sometimes returns them anyway. That makes Sharegate unable to read some messages from SharePoint so it is unable to finish some operations. These characters are often found in the title or in the description of the item being migrated. Most characters should not cause the problem but some of them, like the vertical tab, will cause an error. If this happens on a list or site, it is usually in the title or description but, for a list, for example, it can be in the title of a column.
Look in the migration report and check the items that have this error. Find them in your SharePoint environment and check the title, description and possibly column names to make sure there are no special characters that might not be supported.
The most frequent character is the vertical tab which is not visible but it makes it possible to have text on two lines like if you would use enter to write on a new line. So if you see text that are on multiple lines but usually is not, there is a good chance that you have a vertical tab. We suggest rewriting the text from scratch if you find a problem to make sure to eliminate any illegal characters.