I have the same problem of being unable to add tags in 3.0.6 (and the 3.0.7 upgrade hasn't been rolled out to my servers yet).
The error log when I try to access the tag through the admin settings, and then from the sidebar (add tag), respectively, are below:
Session "default" started at 2015-02-21T10:02:44+0000
#1 ERROR: Error: Invalid argument supplied for foreach() in '/home/lawchar/public_html/application/views/more/section1.php' on line 46 (error code: 2)
#2 ERROR: Error: Undefined index: task in '/home/lawchar/public_html/application/views/more/section1.php' on line 81 (error code:
#3 ERROR: Error: Undefined variable: add_first_obj_onclick in '/home/lawchar/public_html/application/views/more/section1.php' on line 104 (error code:
Time since start: 0.17320895195 seconds
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Session "default" started at 2015-02-21T10:03:04+0000
#1 ERROR: Error: Cannot modify header information - headers already sent by (output started at /home/lawchar/public_html/application/functions.php:1937) in '/home/lawchar/public_html/application/layouts/json.php' on line 1 (error code: 2)
Time since start: 0.397472143173 seconds
-------------------------------------------------------------------------------
Session "default" started at 2015-02-21T10:03:31+0000
#1 ERROR: Error: Undefined variable: selected_members in '/home/lawchar/public_html/application/views/member/add.php' on line 209 (error code:
Time since start: 0.287874221802 seconds
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I'm not entirely sure what the middle error is, I literally go straight from the tags in the admin menu (which then gives the 500 error) to the tags in the sidebar. This brings up the "add new tag" page, but when you hit "add tag" the 500 error appears again.
Hope that this helps get to the bottom of the problem.
Server stats are:
Apache version 2.2.26
PHP version 5.3.29
MySQL version 5.5.40-cll
Architecture x86_64
Operating system linux
