Hi, I see this issue is registered more then once. For example:
Can't Login to Piwik: Form security failed - #42 by brynn or
Can't log in - "Error: Form security failed." RESOLVED - #4 by Lukas or
Can't log in after update to version 3.8.0
mod_proxy Form security failed
But what the solution is, is every time different. 
Solutions mentioned are:
- deleting the tmp-file,
- deleting the cookies,
- changing a parameter in the config.ini,
- edit the something in .htaccess or
- change some code.
But what the reason is, is not clear for me…