In SDL Web 8.1/8.5, you cannot modify components having blank spaces in any valid attribute like title, alt etc.

You have many components in SDL Tridion 2011 or SDL Tridion 2013 SP1 where you have given spaces in RTF fields.
Example :
 

<img title=" " src="" alt=" " class="" width="250" height="9"/>.
<div title= " ">test</div>

You have migrated these components to SDL Web 8.1 or SDL Web 8.5.
When your users then try to modify them you then get a 'the following error message

Unable to Save item
Error: Length cannot be less than zero.
 
 

Solution: Apply hotfix CM_8.1.1.3618 for Web 8.1  Apply hotfix CM_8.5.0.6703 for Web 8.5.  

To get hotfixes, see the following article:

How to get the SDL Web Hotfixes?