[TYPO3] Edit plugin objects and template results in server error

[Axel]

Hi,

I've got a problem with my typo3 installation. Whenever I try and edit 
some of the plugins that I added as page content in the backend of my 
installation I get an error message 500 from my server:

  ------------------------------
  Forbidden
  You don't have permission to access /typo3/alt_doc.php on this server.
  ------------------------------

e.g. by calling
/typo3/alt_doc.php?returnUrl=%2Ftypo3%2Fsysext%2Fcms%2Flayout%2Fdb_layout.php%3Fid%3D32&edit[tt_content][92,]=edit 
requested from 
http://www.pssrc.org/typo3/sysext/cms/layout/db_layout.php?id=32


However, I don't have that problem with all plugins. It works fine with 
the 'better login-box', the 'front end user registration', the 'user 
listing' and the 'chc forum' extensions. I get the error messages with 
the tt_news and the cal extension.

In addition, I sometimes get error messages like this when I try to add 
an entry to the my general storage page using the sixpack extension:

  -----------------------------
Not Acceptable
An appropriate representation of the requested resource 
/typo3/alt_doc.php could not be found on this server.
  -----------------------------

When I try changing my template record I sometimes get this error 
message (even though it sometimes works):

  -----------------------------
Forbidden
You don't have permission to access /typo3/alt_doc.php on this server.
  -----------------------------

Any help would be greatly appreciated - it's driving me nuts...

Cheers,
Axel

**************
Configuration:
**************
typo3 4.0.4
apache 1.3.37 (unix)
mysql 4.1.21-standard
php 5.2.0
perl 5.8.6

local extensions
tt_address  	1.0.4
cal 	0.14.1
chc_forum 	1.4.5
dr_wiki 	1.2.0
fh_library 	0.0.15
sr_feuser_register 	2.5.4
rtehtmlarea 	1.4.3
loginusertrack 	1.1.0
kb_md5fepw 	0.4.0
newloginbox 	3.0.1
tt_news 	2.4.0
sixpack 	0.3.2
static_info_tables 2.0.2