The panel encountered a problem while loading "OAFIID:TomboyApplet".

Same here under Jaunty 64-bit. Trying to execute it in the command line ($ tomboy) gives:

# code #
[DEBUG]: NoteManager created with note path "/home/nik/.tomboy".
[INFO]: Initializing Mono.Addins

Unhandled Exception: System.InvalidOperationException: Could not read add-in description
  at Mono.Addins.Addin.get_Description () [0x00000]
  at Mono.Addins.AddinSessionService.CheckHostAssembly (System.Reflection.Assembly asm) [0x00000]
  at Mono.Addins.AddinSessionService.ActivateRoots () [0x00000]
  at Mono.Addins.AddinSessionService.Initialize () [0x00000]
  at Mono.Addins.AddinManager.Initialize (System.String configDir) [0x00000]
  at Tomboy.AddinManager.InitializeMonoAddins () [0x00000]
  at Tomboy.AddinManager..ctor (System.String tomboy_conf_dir) [0x00000]
  at Tomboy.NoteManager.CreateAddinManager () [0x00000]
  at Tomboy.NoteManager..ctor (System.String directory, System.String backup_directory) [0x00000]
  at Tomboy.NoteManager..ctor (System.String directory) [0x00000]
  at Tomboy.Tomboy.Main (System.String[] args) [0x00000]
# end of code #

Best regards, Nikos

Below more info when trying to run tomboy as root:

--%<--- code --%<--- code --%<--- code --%<---
$ sudo tomboy

(/usr/lib/tomboy/Tomboy.exe:7253): GLib-GObject-CRITICAL **: /build/buildd/glib2.0-2.20.1/gobject/gtype.c:2458: initialization assertion failed, use IA__g_type_init() prior to this function

(/usr/lib/tomboy/Tomboy.exe:7253): GLib-GObject-CRITICAL **: g_object_new: assertion `G_TYPE_IS_OBJECT (object_type)' failed

(/usr/lib/tomboy/Tomboy.exe:7253): GLib-GObject-CRITICAL **: g_object_ref: assertion `G_IS_OBJECT (object)' failed
Stacktrace:

  at (wrapper managed-to-native) GConf.Client.gconf_client_get_default () <0x00050>
  at (wrapper managed-to-native) GConf.Client.gconf_client_get_default () <0xffffffff>
  at GConf.Client..ctor () <0x00053>
  at Tomboy.GConfPreferencesClient..ctor () <0x0002f>
  at Tomboy.GnomeFactory.CreatePreferencesClient () <0x0001f>
  at Tomboy.Services..cctor () <0x00075>
  at (wrapper runtime-invoke) System.Object.runtime_invoke_void (object,intptr,intptr,intptr) <0xffffffff>
  at Tomboy.Application.Initialize (string,string,string,string[]) <0xffffffff>
  at Tomboy.Application.Initialize (string,string,string,string[]) <0x00020>
  at Tomboy.Tomboy.Main (string[]) <0x000c3>
  at (wrapper runtime-invoke) Tomboy.Tomboy.runtime_invoke_void_string[] (object,intptr,intptr,intptr) <0xffffffff>

Native stacktrace:

 mono [0x429e65]
 mono [0x44ba7d]
 /lib/libpthread.so.0 [0x7f84fa41c080]
 /usr/lib/libgconf-2.so.4 [0x7f84f7f566c1]
 /usr/lib/libgconf-2.so.4(gconf_client_get_default+0xc4) [0x7f84f7f5a3c4]
 [0x41d41a70]
39 ../sysdeps/unix/sysv/linux/x86_64/syscall.S: No such file or directory.

Debug info from gdb:

(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
[Thread debugging using libthread_db enabled]
[New Thread 0x7f84fb0e9730 (LWP 7253)]
[New Thread 0x7f84f8cd1950 (LWP 7255)]
[New Thread 0x7f84fb0f9950 (LWP 7254)]
syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:39
 in ../sysdeps/unix/sysv/linux/x86_64/syscall.S
Current language: auto; currently asm
  3 Thread 0x7f84fb0f9950 (LWP 7254) 0x00007f84fa41b7e1 in nanosleep () from /lib/libpthread.so.0
  2 Thread 0x7f84f8cd1950 (LWP 7255) pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
  1 Thread 0x7f84fb0e9730 (LWP 7253) syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:39

Thread 3 (Thread 0x7f84fb0f9950 (LWP 7254)):
#0 0x00007f84fa41b7e1 in nanosleep () from /lib/libpthread.so.0
#1 0x0000000000503be2 in ?? ()
#2 0x00007f84fa4143ba in start_thread (arg=<value optimized out>) at pthread_create.c:297
#3 0x00007f84f9efbfcd in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:112
#4 0x0000000000000000 in ?? ()

Thread 2 (Thread 0x7f84f8cd1950 (LWP 7255)):
#0 pthread_cond_wait@@GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:261
#1 0x0000000000506815 in ?? ()
#2 0x0000000000508f8f in ?? ()
#3 0x000000000052123d in ?? ()
#4 0x0000000000496d33 in ?? ()
#5 0x00000000004b52d3 in ?? ()
#6 0x000000000051d81b in ?? ()
#7 0x000000000053a1c2 in ?? ()
#8 0x00007f84fa4143ba in start_thread (arg=<value optimized out>) at ...

Currently I use tomboy 0.15 (compiled/installed from source) which runs fine. I miss the tomboy-applet for gnome-panel so I created a custom launcher.

Thanks for the report Seif, is this still an issue with Karmic?

I have the same problem here.
Using Ubuntu-Studio 9.10 Karmic 32-bit downloaded 5 days ago.
No solution yet.