This is very likely a problem with [mod_security](https://www.inmotionhosting.com/support/security/what-is-modsecurity-and-why-is-it-important/)
.
It’s probably triggering a security rule set by your host.
You may be able to bypass it by using < for the left bracket, or the host might disable the rule or mod_security itself if you ask. Sometimes you can turn it off yourself in .htaccess with this code:
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
Worst case, you can paste the code into the snippet
field of the chunk in the DB.