Nobody can tell you anything without proper system info or other technical details like version of AE, language settings, text formatting, fonts used etc..
Thanks, of course:
iMac running on OS X (10.11.6) El Capitan
Processor: 4GHz IntelCor i7
After Effects version 2015.3 (126.96.36.199)
Maximum Disk Cache size set on: 300GB
Available space on internal harddisk: 1.2 TB
Language setting in English
I'm also using FontExplorer X Pro
The problem appears using any font.
Hope this helps you out
It appears your clipboard is busy. This could be related to a software you installed and a font software would be my primary suspect. I would reinstall it and check.
The thing is that it's only a problem with AE, not with other applications. I can copy and paste text from and to any other application.
I've tried both, unfortunately the problem isn't solved.
I did find out, that if I copy a color # in Illustrator and past it into text color or layer color in AE, it does work.
So it has something to do with text layer
It might help if you tried to paste the data to a text editor work and provided the dump here or a screenshot of it, but with special characters visible (spaces, line breaks etc.). I smell another variation of the "OSX inserts extra characters" issue here. Roei does have a point, though. Check if not some otehr tool is usurping your data and possible reformating it in a way it gets invalidated for AE. Perhaps you have some messenger app running that tries to interpret it and scours for hashtags, links, user names from your buddy list or any of that. Likewise, automatic spell checking, translation or font tools can mess things up.