The mail.app (3.6) crashes on launch -- or rather, it doesn't launch. The report says:
Process: Mail  Path:
/Applications/Mail.app/Contents/MacOS/Mail Identifier: com.apple.mail Version: ??? (???) Build Info: Mail-9360000~1 Code Type: PPC (Native) Parent Process: launchd 
Interval Since Last Report:
3170 sec Crashes Since Last Report:
1 Per-App Interval Since Last Report: 0 sec Per-App Crashes Since Last Report: 1
Date/Time: 2010-01-24 01:38:35.221 +0100 OS Version:
Mac OS X 10.5.8 (9L31a) Report Version: 6 Anonymous UUID: 8323A671-FCB6-4393-A60E-F633543786FF
Exception Type: EXC_BREAKPOINT (SIGTRAP) Exception Codes: 0x0000000000000001, 0x000000008fe0105c Crashed Thread: 0
Dyld Error Message: Symbol not found: __NSConcreteStackBlock
Referenced from: /System/Library/PrivateFrameworks/iLifeMediaBrowser.framework/Versions/A/iLifeMediaBrowser Expected in: /usr/lib/libSystem.B.dylib
The system.log says (don't know if it's related; there are sevral passages like this one):
Jan 24 01:21:52 carl-henrik-fredrikssons-imac-g5 com.apple.dyld: update_dyld_shared_cache for arch=ppc failed: could not resolve _DASessionSetDispatchQueue from /System/Library/PrivateFrameworks/iLifeMediaBrowser.framework/Versions/A/iLifeMediaBrowser in /System/Library/PrivateFrameworks/iLifeMediaBrowser.framework/Versions/A/iLifeMediaBrowser Jan 24 01:21:54 carl-henrik-fredrikssons-imac-g5 com.apple.dyld: update_dyld_shared_cache for arch=ppc failed: read failure copying dylib. Read of 4816896 bytes at file offset 45056 returned 222592 for /System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/CoreGraphics.framework/Versions/A/Resources/libCGATS.A.dylib while copying /System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/CoreGraphics.framework/Versions/A/Resources/libCGATS.A.dylib to shared cache
There's only one Mail.app on the computer.
I have two users on my machine (iMac G5, iSight, Mac OS 10.5.8). Same problem with both users. I have tried to delete all mail-related files in the library (e.g. Mail folder and com.apple.mail.plist).
I have also repaired permissions and installed the combo update of 10.5.8.
Before I started the Mail.app for the first time, I migrated programs and one user profile from another mac (MacBook Pro running Snow Leopard). I guess this might have caused the problem.
I don't have a Leopard system disc (the OS was installed on the hard drive when I had to have it (the hard drive) replaced by an official Apple shop. I don't know what more to try...