Downgrading (user homes) to macOS Sierra Because of having issues with High Sierra, I had to downgrade to Sierra. My macOS is on an internal SSD (of my Mac Pro) and my user homes are on an external SSD. When downgrading using a backup, Time Machine only replaced macOS (on the internal SSD), but not the user homes (on the external SSD). 

Looks like Sierra is not happy with my High Sierra user homes:


Messages hangs
identityservicesd and callservicesd are constantly crashing
my screen turns black, sometimes every few seconds. possibly having to do with the crashes. 


Essentially the same problems like in this thread (which is locked): 
https://discussions.apple.com/thread/8105075

The user of the post above said he could solve his issues with a reinstall of Sierra (which I cannot do and I am also not convinced this changes anything) plus “restoring files by hand from my time machine backup drive with rsync and other command-line tools”. Any idea how to do this or which files need replacement? Deleting ~/Library/Caches did not help. I also restored these with Time Machine which also did not help:


~/Library/Messages
~/Library/Preferences/com.apple.identityservicesd.plist
~/Library/Preferences/com.apple.TelephonyUtilities.plist


Any other idea?

Some detail on the crashes:

Process:               identityservicesd [803]
Path:                  /System/Library/PrivateFrameworks/IDS.framework/identityservicesd.app/Contents/MacOS/identityservicesd
Identifier:            identityservicesd
Version:               10.0 (1000)
Code Type:             X86-64 (Native)
Parent Process:        ??? [1]
Responsible:           identityservicesd [803]
User ID:               501

Date/Time:             2018-02-01 10:00:56.010 +0100
OS Version:            Mac OS X 10.12.6 (16G1212)
Report Version:        12
Anonymous UUID:        F9C97F8F-70BD-CA59-5E14-105C64D05D82


Time Awake Since Boot: 57 seconds

System Integrity Protection: enabled

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_CRASH (SIGABRT)
Exception Codes:       0x0000000000000000, 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY

Application Specific Information:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFData count]: unrecognized selector sent to instance 0x7fa61a012200'
terminating with uncaught exception of type NSException
abort() called


and

Process:               callservicesd [996]
Path:                  /System/Library/PrivateFrameworks/TelephonyUtilities.framework/callservicesd
Identifier:            callservicesd
Version:               1.0 (1.0)
Code Type:             X86-64 (Native)
Parent Process:        ??? [1]
Responsible:           callservicesd [996]
User ID:               501

Date/Time:             2018-02-01 10:09:25.222 +0100
OS Version:            Mac OS X 10.12.6 (16G1212)
Report Version:        12
Anonymous UUID:        F9C97F8F-70BD-CA59-5E14-105C64D05D82


Time Awake Since Boot: 560 seconds

System Integrity Protection: enabled

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_CRASH (SIGABRT)
Exception Codes:       0x0000000000000000, 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY

Application Specific Information:
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFData count]: unrecognized selector sent to instance 0x7fd24b86e800'
terminating with uncaught exception of type NSException
abort() called


Update: Trying to restore ~/Library/Preferences completely with Time Machine essentially resulted in a deletion. Now the problems are gone but my preferences are also gone. Luckily I have copied the folder before doing so. Thus if I knew which of the preferences needed replacement by the Sierra ones I could restore the rest by hand. Any idea?
It will be very difficult to isolate the issue to individual preference files, since there could be several subtle issues.

Better to take the opposite approach - start with no preferences and only copy the old preferences over one by one as you come across them. That is, as you discover that you have a preference that needs updating, find the single preference file that will give you that preference, and restore it. Re-test and only proceed if things are still stable. Will be a drawn out process, but necessarily so I think. When you can no longer add individual preferences, consider the restoration finished and many set the remaining preferences.

This complex relationship between the OS, Applications and user files is why Apple created Migration Assistant. It might not be help here but it is useful to know just how intricate the process is of "migrating" from one OS version to another.