All plugins fail to activate

Lets fix this systematically.

Deactivate all plugins

Activate first plugin; post errors here; so we can help

Activ. next plug; post its SPECIFIC error here, then we help with that, etc….

Thank you for your time.

Step-by-step:
-Deactivated all plugins on the main site.
-Installed Plugin:My-Calendar (I had deleted it while trying multiple plugins)
-I notice that on the dashboard for the main page, it says that I am using “2 Widgets”. All widgets are deactivated. I confirmed this by checking the Appearance > Widgets panel.
-Activate plugin:My-Calendar on main site. In the admin panel, the following is displayed at the top of every page. There are also single notices about line 251 (as below) in various other places in the admin panel. Ask if you would like a list.

Notice: Undefined index: page in /home/content/98/7276698/html/wp-content/plugins/my-calendar/my-calendar.php on line 251

Notice: Undefined index: page in /home/content/98/7276698/html/wp-content/plugins/my-calendar/my-calendar.php on line 256

Notice: Undefined index: page in /home/content/98/7276698/html/wp-content/plugins/my-calendar/my-calendar.php on line 342

Notice: Undefined index: page in /home/content/98/7276698/html/wp-content/plugins/my-calendar/my-calendar.php on line 342

Notice: Undefined index: page in /home/content/98/7276698/html/wp-content/plugins/my-calendar/my-calendar.php on line 342

[...]

(There are a total of 7 notices about line 342.)

I’ll try to split this up for readability.

-Activate plugin:Most Commented Widget (Customized edit)
-No errors.
-Deactivate plugin:Most Commented Widget (Customized edit)

(oops. Apparently it’s not all widgets. Sorry. I thought it was because I have tried multiple calendar widgets, and they all fail.)

-Activate plugin:Grunion Contact Form
-No errors.
-Deactivate plugin:Grunion Contact Form

-Activate plugin:Event Calendar 3 for PHP 5.3
-Activates, but error at top of page while viewing plugins panel:

Notice: load_plugin_textdomain was called with an argument that is deprecated since version 2.7 with no alternative available. in /home/content/98/7276698/html/wp-includes/functions.php on line 3389

Notice: date_default_timezone_set() [function.date-default-timezone-set]: Timezone ID 'UTC+7' is invalid in /home/content/98/7276698/html/wp-content/plugins/event-calendar-3-for-php-53/tz.php on line 27

Notice: date_default_timezone_set() [function.date-default-timezone-set]: Timezone ID 'UTC+7' is invalid in /home/content/98/7276698/html/wp-content/plugins/event-calendar-3-for-php-53/tz.php on line 27

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/content/98/7276698/html/wp-includes/functions.php on line 3387

And above the plugins, wordpress reports:
The plugin generated 1171 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

-Activate Plugin:Event Calendar 3 for PHP 5.3 Widget
-Error: White screen displaying the following:

Notice: load_plugin_textdomain was called with an argument that is deprecated since version 2.7 with no alternative available. in /home/content/98/7276698/html/wp-includes/functions.php on line 3389

Notice: date_default_timezone_set() [function.date-default-timezone-set]: Timezone ID 'UTC+7' is invalid in /home/content/98/7276698/html/wp-content/plugins/event-calendar-3-for-php-53/tz.php on line 27

Notice: date_default_timezone_set() [function.date-default-timezone-set]: Timezone ID 'UTC+7' is invalid in /home/content/98/7276698/html/wp-content/plugins/event-calendar-3-for-php-53/tz.php on line 27

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/content/98/7276698/html/wp-includes/functions.php on line 3387

Warning: Cannot modify header information - headers already sent by (output started at /home/content/98/7276698/html/wp-includes/functions.php:3389) in /home/content/98/7276698/html/wp-includes/pluggable.php on line 897

Warning: Cannot modify header information - headers already sent by (output started at /home/content/98/7276698/html/wp-includes/functions.php:3389) in /home/content/98/7276698/html/wp-includes/pluggable.php on line 897

-I’m able to get back to the panel by navigating back in my history. Deactivate plugin:Event Calendar 3 for PHP 5.3
-Deactivates, but error at top of page:

Notice: register_widget_control is deprecated since version 2.8! Use wp_register_widget_control() instead. in /home/content/98/7276698/html/wp-includes/functions.php on line 3303

Notice: register_widget_control is deprecated since version 2.8! Use wp_register_widget_control() instead. in /home/content/98/7276698/html/wp-includes/functions.php on line 3303

-Deactivate plugin:Event Calendar 3 for PHP 5.3 Widget
-No errors.

What theme are you using try and switch to default twenty ten theme and see if you still the problem if you do

then

Delete all files and folders except wp-content folder and wp-config.php and replace them from fresh wordpress download.

>>Warning: Cannot modify header information – headers already sent by

These are nearly serious errors

I would not call the notices errors; did you ever install a clean WP and see how many of these (minor) notices there are

The problem is resolved.

Solution: it’s still a little unclear, but I believe that replacing the wordpress files as per govpatel worked. Either that, or the mere fact that I had debug mode on was causing problems. Or both.

Thank you to all who have helped!

You are welcome