Hey Timo. It's quite an old machine but somehow still runs miraculously fast for 99% of apps. And the only way I can play my 32-bit version of Unreal Tournament...
Perhaps this helps? Let me know if you need more info and happy to send you the full crash report via email.
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[NSFont monospacedSystemFontOfSize:weight:]: unrecognized selector sent to class 0x7fff9e756078'
abort() called
terminating with uncaught exception of type NSException
Application Specific Backtrace 1:
0 CoreFoundation 0x00007fff3d0a7b79 __exceptionPreprocess + 256
1 libobjc.A.dylib 0x00007fff678403c6 objc_exception_throw + 48
2 CoreFoundation 0x00007fff3d1221cb __CFExceptionProem + 0
3 CoreFoundation 0x00007fff3d049658 ___forwarding___ + 1478
4 CoreFoundation 0x00007fff3d049008 _CF_forwarding_prep_0 + 120
5 ??? 0x0000000118cee984 0x0 + 4711180676
6 Xamarin.Mac.dll.dylib 0x000000010a6b3f19 AppKit_NSFont_MonospacedSystemFont_System_nfloat_System_nfloat + 57
7 WorkingHours.exe.dylib 0x0000000109a37e87 WorkingHours_PomodoroViewController_ViewDidLoad + 103
8 ??? 0x000000010de50df1 0x0 + 4528082417
9 WorkingHours 0x00000001057b257e mono_jit_runtime_invoke + 1550
10 WorkingHours 0x00000001058e7688 mono_runtime_invoke_checked + 136
11 WorkingHours 0x00000001058ebf5e mono_runtime_invoke + 142
12 WorkingHours 0x00000001059d6e36 _ZL30native_to_managed_trampoline_2P11objc_objectP13objc_selectorPP11_MonoMethodj + 246
13 WorkingHours 0x0000000105a10f4c -[PomodoroViewController viewDidLoad] + 44
14 AppKit 0x00007fff3a75dbaf -[NSViewController _sendViewDidLoad] + 94
15 AppKit 0x00007fff3a75b0eb _noteLoadCompletionForObject + 646