I finally got both my iMac Late 2015 and my MacBook Pro 2016 booting properly again. I had done the security update 2018-002 10.13.6 via App Store yesterday (Nov. 3rd). On both machines the update seemed to work fine and first booted with login screen. I would otherwise not have done the other machine...
However, this morning both Macs booted very slowly and ended with a "Installer Crash Log Viewer", indicating the update hadn't completed.
I managed to reinstall High Sierra from Recovery mode. Here is part of the crashreport:
Nov 4 12:20:51 iMac ReportCrash[553]: Process: macOS Installer [548]
Nov 4 12:20:51 iMac ReportCrash[553]: Path: /System/Installation/CDIS/macOS Installer.app/Contents/MacOS/macOS Installer
Nov 4 12:20:51 iMac ReportCrash[553]: Identifier: com.apple.macOS-Installer
Nov 4 12:20:51 iMac ReportCrash[553]: Version: 1.0 (1006)
Nov 4 12:20:51 iMac ReportCrash[553]: Build Info: OSInstaller-1006000000000000~88
Nov 4 12:20:51 iMac ReportCrash[553]: Code Type: X86-64 (Native)
Nov 4 12:20:51 iMac ReportCrash[553]: Parent Process: ??? [529]
Nov 4 12:20:51 iMac ReportCrash[553]: Responsible: macOS Installer [548]
Nov 4 12:20:51 iMac ReportCrash[553]: User ID: 0
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: Date/Time: 2018-11-04 04:20:40.624 -0800
Nov 4 12:20:51 iMac ReportCrash[553]: OS Version: Mac OS X 10.13.6 (17G3025)
Nov 4 12:20:51 iMac ReportCrash[553]: Report Version: 12
Nov 4 12:20:51 iMac ReportCrash[553]: Anonymous UUID: D48F6FF9-68BF-4982-B2F4-C86878B1C02A
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: Time Awake Since Boot: 150 seconds
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: System Integrity Protection: disabled
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Nov 4 12:20:51 iMac ReportCrash[553]: Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000018
Nov 4 12:20:51 iMac ReportCrash[553]: Exception Note: EXC_CORPSE_NOTIFY
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: Termination Signal: Segmentation fault: 11
Nov 4 12:20:51 iMac ReportCrash[553]: Termination Reason: Namespace SIGNAL, Code 0xb
Nov 4 12:20:51 iMac ReportCrash[553]: Terminating Process: exc handler [0]
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: VM Regions Near 0x18:
Nov 4 12:20:51 iMac ReportCrash[553]: -->
Nov 4 12:20:51 iMac ReportCrash[553]: __TEXT 00000001017fa000-0000000101811000 [ 92K] r-x/rwx SM=COW /System/Installation/CDIS/macOS Installer.app/Contents/MacOS/macOS Installer
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: Application Specific Information:
Nov 4 12:20:51 iMac ReportCrash[553]: objc_msgSend() selector name: release
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]:
Nov 4 12:20:51 iMac ReportCrash[553]: Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
Nov 4 12:20:51 iMac ReportCrash[553]: 0 libobjc.A.dylib 0x000000010291ae9d objc_msgSend + 29
Nov 4 12:20:51 iMac ReportCrash[553]: 1 com.apple.macOS-Installer 0x00000001017fc840 0x1017fa000 + 10304
Nov 4 12:20:51 iMac ReportCrash[553]: 2 libdispatch.dylib 0x00000001075525fa _dispatch_call_block_and_release + 12
Nov 4 12:20:51 iMac ReportCrash[553]: 3 libdispatch.dylib 0x000000010754adb8 _dispatch_client_callout + 8
Nov 4 12:20:51 iMac ReportCrash[553]: 4 libdispatch.dylib 0x0000000107556395 _dispatch_main_queue_callback_4CF + 1148
Nov 4 12:20:51 iMac ReportCrash[553]: 5 com.apple.CoreFoundation 0x0000000104833d49 __CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE__ + 9
Nov 4 12:20:51 iMac ReportCrash[553]: 6 com.apple.CoreFoundation 0x00000001047f651a __CFRunLoopRun + 2586
Nov 4 12:20:51 iMac ReportCrash[553]: 7 com.apple.CoreFoundation 0x00000001047f5867 CFRunLoopRunSpecific + 487
Nov 4 12:20:51 iMac ReportCrash[553]: 8 com.apple.HIToolbox 0x000000010d587d96 RunCurrentEventLoopInMode + 286
Nov 4 12:20:51 iMac ReportCrash[553]: 9 com.apple.HIToolbox 0x000000010d587b06 ReceiveNextEventCommon + 613
Nov 4 12:20:51 iMac ReportCrash[553]: 10 com.apple.HIToolbox 0x000000010d587884 _BlockUntilNextEventMatchingListInModeWithFilter + 64
Nov 4 12:20:51 iMac ReportCrash[553]: 11 com.apple.AppKit 0x0000000102ef7a73 _DPSNextEvent + 2085
Nov 4 12:20:51 iMac ReportCrash[553]: 12 com.apple.AppKit 0x000000010368de34 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044
Nov 4 12:20:51 iMac ReportCrash[553]: 13 com.apple.AppKit 0x0000000102eec885 -[NSApplication run] + 764
Nov 4 12:20:51 iMac ReportCrash[553]: 14 com.apple.AppKit 0x0000000102ebba72 NSApplicationMain + 804
Nov 4 12:20:51 iMac ReportCrash[553]: 15 com.apple.macOS-Installer 0x00000001017fc0ef 0x1017fa000 + 8431
Nov 4 12:20:51 iMac ReportCrash[553]: 16 libdyld.dylib 0x00000001075be015 start + 1