With version 0.9.8.4 Thunderbird works.
From version 1.0.0.0 it does not work.
I just tested it again and emptied the sandbox folder each time.
But: I have a special configuration [browser] for Firefox (91.5.0 ESR) and for Thunderbird. Firefox continues to work in this browser-box, only Thunderbird does not.
In the default-box Thunderbird also starts. So it will be because of the configuration, as you suspected.... But what could it be?
These are the corresponding sections in my Sandboxie.ini:
[GlobalSettings]
BoxNameTitle=Y
Template=VirtualDesktopManager
Template=WindowsLive
Template=7zipShellEx
Template=LogitechSetPoint
Template=WindowsRasMan
Template=ComodoInternetSecurity
Template=OfficeLicensing
Template=WacomTablet
ActivationPrompt=n
ExperimentalProtection64Bit=y
FileRootPath=D:\Sandbox\%USER%\%SANDBOX%
SeparateUserFolders=y
KeyRootPath=\REGISTRY\USER\Sandbox_%USER%_%SANDBOX%
IpcRootPath=\Sandbox\%USER%\%SANDBOX%\Session_%SESSION%
NetworkEnableWFP=n
EditAdminOnly=n
ForceDisableAdminOnly=n
ForgetPassword=n
StartRunAlertDenied=n
NotifyStartRunAccessDenied=y
[UserSettings_Portable]
SbieCtrl_EnableLogonStart=y
SbieCtrl_EnableAutoStart=n
SbieCtrl_AddDesktopIcon=n
SbieCtrl_AddQuickLaunchIcon=n
SbieCtrl_AddContextMenu=y
SbieCtrl_AddSendToMenu=y
SbieCtrl_Hidden=Y
SbieCtrl_ActiveView=40021
SbieCtrl_UserName=Portable
SbieCtrl_ShowWelcome=N
SbieCtrl_ReSyncContextMenu=N
SbieCtrl_NextUpdateCheck=-1
SbieCtrl_UpdateCheckNotify=y
SbieCtrl_AutoApplySettings=y
SbieCtrl_HideWindowNotify=n
SbieCtrl_ShouldDeleteNotify=N
SbieCtrl_EditConfNotify=n
SbieCtrl_RecoverTarget=C:\Users\MrZ\Desktop
SbieCtrl_SaveRecoverTargets=y
SbieCtrl_ShortcutNotify=n
SbieCtrl_WindowCoords=477,224,660,576
SbieCtrl_ProcessViewColumnWidths=250,70,310
SbieCtrl_AutoStartAgent=SandMan.exe
[Browser]
Enabled=y
ConfigLevel=9
AutoRecover=y
OpenFilePath=thunderbird.exe,D:\Eigene Dateien\Office\Thunderbird\AppData Roaming\Thunderbird\
OpenFilePath=%Desktop%\
OpenFilePath=D:\Eigene Dateien\Desktop\
OpenFilePath=firefox.exe,%AppData%\Mozilla\Firefox\Profiles\00000001.Standard_01\places.sqlite
OpenFilePath=firefox.exe,D:\Anwendungsdaten\Firefox\Profile\Mozilla (AppData Roaming)\Firefox\Profiles\00000001.Standard_01\places.sqlite
OpenFilePath=firefox.exe,D:\Anwendungsdaten\Firefox\Profile\Mozilla (AppData Roaming)\Firefox\Profiles\00000001.Standard_01\bookmarkbackups\
Template=SkipHook
Template=FileCopy
Template=Firefox_Bookmarks_DirectAccess
Template=BlockPorts
Template=LingerPrograms
Template=Firefox_Phishing_DirectAccess
Template=AutoRecoverIgnore
CopyLimitKb=307200
BorderColor=#80ff80,off,6
DropAdminRights=y
BoxNameTitle=y
ClosedFilePath=<BlockNetDevices>,InternetAccessDevices
HideOtherBoxes=y
[DefaultBox]
ConfigLevel=9
AutoRecover=y
RecoverFolder=%Desktop%
Enabled=y
Template=SkipHook
Template=FileCopy
Template=BlockPorts
Template=LingerPrograms
Template=Firefox_Phishing_DirectAccess
Template=RoboForm
Template=AutoRecoverIgnore
CopyLimitKb=307200
BorderColor=#ff0000,on,6
DropAdminRights=y
BoxNameTitle=y
ClosedFilePath=<BlockNetDevices>,InternetAccessDevices
HideOtherBoxes=y