My problem magically solved itself this morning :-)
What I did to manage another problem with the ActivityLog (thanks to Kimberley :-) that I couldn't reset was to upload my local /tmpl/cms/ and overwrite the /tmpl/cms on the server.
That did the right trick with my ActivityLog bug, but to my amazement when I started working on the MT-Notifier again, that plugin was working perfect too.
Now I don't like things that don't work, but I really hate problems that dissappear by themselvers, cause you never know when they will come back, only that they will come back. Now I don't have to scratch my head anymore.

But thanks to Katymum:

[CODE] Hi Leon,I noticed in another thread that you are using MT-Notifier. I recently upgraded MT-Notifier and experienced the exact same problem you described. There is a file in that plugin (tmpl/cms/view_log.tmpl) that overwrites MT's file of the same name. Unfortunately, MT-Notifier's view_log.tmpl does not include the "magic token" info that MT 3.16 looks for. That's why re-uploading MT's version fixed your problem. [/CODE]

Two probnlems solved with one upload :-)

Another trick I found out myself regarding MT-Notofier; maybe useful to others.
I've been using an old mailmachine.cgi for years, that was working with a flat file, one e-mail adress per line.
How to bulk import those adress in MT-Notifier? MT-Notifier is able to import them from the old MT Notification List, so the trick is to import that flat file into that one to begin with.
First add at least one adress to the old Notification List (I'didn't have any notifications over there) and then export the mt_notification table from the mt database.

You get this format:

[CODE]
INSERT INTO `mt_notification` VALUES 1, 1, NULL, 'mail@krijnen.com', '', '2005-05-31 11:01:23', '20050531110123', NULL, NULL);
[/CODE]

With the e-mail adress in the center of the line of code, take a good editor - I used TextPad - and do some sorting, extended search and replacing with regular expressions, so that your file becomes like this:

[CODE]
INSERT INTO `mt_notification` VALUES 2, 1, NULL, 'mail1@gmail.com', '', '2005-05-31 11:01:23', '20050531110123', NULL, NULL);
INSERT INTO `mt_notification` VALUES 3, 1, NULL, 'mail2@gmail.com', '', '2005-05-31 11:01:23', '20050531110123', NULL, NULL);
INSERT INTO `mt_notification` VALUES 4, 1, NULL, 'mail3@gmail.com', '', '2005-05-31 11:01:23', '20050531110123', NULL, NULL);
INSERT INTO `mt_notification` VALUES 5, 1, NULL, 'mail4@gmail.com', '', '2005-05-31 11:01:23', '20050531110123', NULL, NULL);
[/CODE]

The notification_id is the primary key over here, so let VALUES grow when sorting in your editor, or do it by hand.

Import this text file into the mt_notification table - only cutting and pasting if you use PHPMyAdmin, and all adresses (I had several hundred) show up in the old Notification List.
Now go to the MT-Notifier plugin, click 'transfer', and choose 'Movable Type Notifications' in the menu. After a few seconds (in this case a couple of hundred adresses) the Notifier plugin is ready importing and you can manage all adresses now from the plug-in. If you want you can leave the old table in MySQL where it is. It doesn't hurt, and can serve, if you like, as an extra back-up. But remember: ALWAYS export your MT entries before you do things like this, and ALWAYS dump your MySQL tables first, to stay on the safer side of things.

Hi, Leon! Seeing this message means the files in the /tmpl/cms/ directory have not been updated to the latest versions.You should upload all of the 3.16 files in /tmpl/cms/ again, ensuring that the files are all transferred in ASCII mode and that they are completely transferred, overwriting the older versions on the server. Kymberlie

Hi Leon,I noticed in another thread that you are using MT-Notifier. I recently upgraded MT-Notifier and experienced the exact same problem you described. There is a file in that plugin (tmpl/cms/view_log.tmpl) that overwrites MT's file of the same name. Unfortunately, MT-Notifier's view_log.tmpl does not include the "magic token" info that MT 3.16 looks for. That's why re-uploading MT's version fixed your problem. However, in order to view the log properly (with MT-Notifier's activity) you will need to put MT-Notifier's version of view_log.tmpl back in there, but with this edit:In MT-Notifier's tmpl/cms/view_log.tmpl, change the corresponding line (31?) to this:

------------ CODE SAMPLE ----------
<form><input type="button" onclick="window.location='<TMPL_VAR NAME=SCRIPT_URL>?__mode=reset_log&magic_token=<TMPL_VAR NAME=MAGIC_TOKEN>'" value="<MT_TRANS phrase="Reset Activity Log">">