I have similar problems with the andreas01 snippet and mod_security on apache.
I could have given this a try locally, but as I happen to have access to my hosting provider’s machines (just happen to have a job there)
I took the opportunity to have a closer look and see what happens when altering things.
After some bad things happening lately mod_security was installed with rulesets from www.gotroot.com.
Some rules already had been taken out to avoid problems with phpMyAdmin but some were still left.
The first error I encountered (ip address and host name have been changed):
[Fri May 5 20:55:42 2006] [error] [client 192.168.1.1] mod_security: Access denied with code 403. Pattern match "(insert[[:space:]]+into.+values|select.+from|bulk[[:space:]]+insert|union.+select)" at POST_PAYLOAD [id "300016"][rev "1"] [msg "Generic SQL injection protection"] [severity "2"] [hostname "www.mydomain.com"] [uri "/manager/index.php?a=24"] [unique_id "RFufrVEEULQAAVkRifU"]
After taking out this rule, I hit a second rule:
[Fri May 5 21:04:41 2006] [error] [client 192.168.1.1] mod_security: Access denied with code 500. Pattern match "(((URL|SRC|HREF|LOWSRC)[\\\\s]*=)|(url[\\\\s]*[\\\\(]))[\\\\s]*[\\\\’\\"]*[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*j[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*a[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*v[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*a[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*s[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*c[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*r[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*i[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*p[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*t[\\\\x09\\\\x0a\\\\x0b\\\\x0c\\\\x0d]*[\\\\:]" at POST_PAYLOAD [hostname "www.mydomain.com"] [uri "/manager/index.php?a=24"] [unique_id "RFuhyFEEULQAAWMfGRU"]
This got me puzzled

, and I tried another snippet and see what happened there:
[Fri May 5 21:51:14 2006] [error] [client 192.168.1.1] mod_security: Access denied with code 403. Pattern match "(chr|fwrite|fopen|system|echr|passthru|popen|proc_open|shell_exec|exec|proc_nice|proc_terminate|proc_get_status|proc_close|pfsockopen|leak|apache_child_terminate|posix_kill|posix_mkfifo|posix_setpgid|posix_setsid|posix_setuid|phpinfo)\\\\(.*\\\\)\\\\;" at POST_PAYLOAD [id "300008"][rev "1"] [msg "Generic PHP exploit pattern denied"] [severity "2"] [hostname "www.mydomain.com"] [uri "/manager/index.php?a=24"] [unique_id "RFusslEEULQAAXsXpz0"]
This was the point where the bell started ringing

It isn’t the actual MODx code that is causing the problems, it is the payload that is hurting.
Maybe the payload should be encoded at the browser, then decoded at the server (I know a bit about programming, can read and modify PHP, and after some reading maybe even JavaScript, but e.g. can JavaScript uuencode, then send, and PHP uudecode then install snippet?)
Technically it would be safer, too.
With bad things happening over and over again, more hosting providers will turn to mod_security (or similar) and sending plain PHP, as in snippets will cause more and more problems.
Regards,
Marb