After some analysis that Christian did, we now believe the crash at launch is not caused by MBS plugins in particular but by Xojo's code that loads a plugin, i.e. any plugin.
We see this kind of crash:
Application Specific Information:
objc_msgSend() selector name: release
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libobjc.A.dylib 0xa6c63240 objc_msgSend + 16
1 com.apple.CoreFoundation 0x9398635f ___CFXRegistrationPost_block_invoke + 105
2 com.apple.CoreFoundation 0x939862b0 _CFXRegistrationPost + 428
3 com.apple.CoreFoundation 0x9398601b ___CFXNotificationPost_block_invoke + 101
4 com.apple.CoreFoundation 0x9395210a -[_CFXNotificationRegistrar find:object:observer:enumerator:] + 1562
5 com.apple.CoreFoundation 0x939513ab _CFXNotificationPost + 891
6 com.apple.Foundation 0x953cc893 -[NSNotificationCenter postNotificationName:object:userInfo:] + 71
7 com.apple.Foundation 0x953d7b8f -[NSNotificationCenter postNotificationName:object:] + 37
8 com.apple.AppKit 0x9157a975 -[NSApplication finishLaunching] + 525
9 rbframework.dylib 0x00458c12 0x357000 + 1055762
10 com.apple.AppKit 0x9157a3dc -[NSApplication run] + 225
11 rbframework.dylib 0x00458c93 0x357000 + 1055891
12 rbframework.dylib 0x003db437 0x357000 + 541751
13 rbframework.dylib 0x003ac4d2 RuntimeRun + 50
14 0x0012c115 REALbasic._RuntimeRun + 34
15 0x00002979 _Main + 245
16 0x000024f0 % main + 36
17 0x001687ac _start + 116
18 0x00168702 start + 43
It seems that the code that loads a plugin (through dlOpen) does not correctly retain some object that is then later supposed to receive a notification.
Christian also found that this crash does not always occur, suggesting it's a race condition where the object in question might get autoreleased too early, or live long enough for the notification to get posted.
Either way, this will break a few tools I and maybe others have released in the past unless we can coerce Apple to add a special case to avoid this particular crash in Xojo-made apps.