Lightroom CC: Migration fails with non-informative error message

I am trying to migrate my catalog from Lightroom Classic CC to the new Lightroom CC and getting a non-helpful error message directly after checking file paths.

SIMILAR REPORTS/CONVERSATIONS (but still not the same)
- https://feedback.photoshop.com/photoshop_family/topics/migration-fails-with-unhelpful-error
- https://feedback.photoshop.com/photoshop_family/topics/lightroom-cc-catalog-migration-error-unknown

MY ENVIRONMENT
- MacBookPro11,3 with MacOS 10.13 with a SSD formatted with encrypted APFS
- Lightroom Classic CC 7.0 [ 1140024 ]
- Lightroom CC 1.0 [ 20170919-1412-ccb76bd ] (Sep 19 2017)
- "Classic"-Catalog at default place /Users//Pictures/Lightroom/Lightroom Catalog.lrcat
- about 30.000 image files (mostly NEF raws and JPEGs) of total size 500 GB
- Image files stored on connected samba share smb://._smb._tcp.local// (running on a Synology NAS with latest firmware).
- CC originals folder location changed to an external spinning disk hard drive (size: 1 TB). Connected via USB. Formatted as MacOS Extended (Journaled, Encrypted).
- free space of main harddrive close to 150 GB

PREPARATIONS
- strictly followed  https://helpx.adobe.com/lightroom-cc/using/migrate-to-lightroom-cc.html
- double checked if samba share and USB disk are connected and accessible/writable

STEPS 
1.) start Lightroom CC via Terminal using option "-tracebacks"
2.) start migrating catalog via UI of Lightroom CC. (File -> Import -> Migrate LR catalog)
3.) migration wizard starts up and checks all file paths. At least the maximum count of my images is reached before the message appears.

OBSERVED BEHAVIOR
- Error message: "Migration was not successful. No one is more unhappy about this than us. Except you probably. Please contact our support team." 
- For Terminal output see end of this post
- Migration is terminated

EXPECTED BEHAVIOR
- Successful migration 😉 or at least some useful information that could help me (or support) to resolve the issue.
- Some log file that could provide more information. Where could I look on what actually went wrong?

TERMINAL OUTPUT
2017-10-23 21:30:08.912 Adobe Lightroom CC[1676:304008] Enabling noisy tracebacks. objc[1678]: Class MacExePath is implemented in both /Library/Application Support/Adobe/OOBE/PDApp/P7/adobe_licutil.app/Contents/MacOS/adobe_licutil (0x1f6454) and /Applications/Utilities/Adobe Application Manager/P7/axlibv7.dylib (0x4c1d1ac). One of the two will be used. Which one is undefined.

2017-10-23 21:30:09.333 Adobe Lightroom CC[1676:304008] *** WARNING: Textured window is getting an implicitly transparent titlebar. This will break when linking against newer SDKs. Use NSWindow's -titlebarAppearsTransparent=YES instead.

2017-10-23 21:30:11.948 Adobe Lightroom CC[1676:304008] Failed to restore first responder (message was _finishedLoadingNibDependencies) NOT KEY

2017-10-23 21:30:25.407 Adobe Lightroom CC[1676:304008] 

maybeAssignLayoutAttributeL had an unhandled key: place

2017-10-23 21:38:20.939 Adobe Lightroom CC Helper[1679:313266] WFModels ERROR Observable model "WFCatUserModel.makeUserModel/catalog/checkNeededMainDriveSpace" posting transient error {

}, "bad argument #2 ((*temporary)) to ? (string expected, got boolean)", nil

2017-10-23 21:38:20.944 Adobe Lightroom CC[1676:313272] WFModels ERROR Observable model "checkNeededMainDriveSpace" posting transient error {

}, "bad argument #2 ((*temporary)) to ? (string expected, got boolean)", nil

2017-10-23 21:38:20.944 Adobe Lightroom CC[1676:304008] 

?:0: attempt to index a nil value

    0: upvalue  ?                              - 749838990:1956+1

    1:          [unnamed]                      - 1096034565:20+3

    2: global   pcall                          - C

    3: method   next                           - 835593318:52+7

    4: method   onNext                         - 1132845634:30+6

    5:          [unnamed]                      - 2135929331:61+8

    6: global   pcall                          - C

    7:          [unnamed]                      - 2135929331:185+20

    8: global   pcall                          - C

    9:          [unnamed]                      - 1802721243:96+14

   10:          [unnamed]                      - tail

   11: global   pcall                          - C

   12: method   invoke                         - 363310666:47+4

   13:          [unnamed]                      - 1220733886:30+14

   14: global   pcall                          - C

   15:          [unnamed]                      - 1220733886:46+24

   16:          [unnamed]                      - tail

   17: method   ensureActive                   - 2135929331:221+23

   18: method   onNext                         - 1333402193:369+38

   19:          [unnamed]                      - 749838990:1848+11

   20:          [unnamed]                      - tail

   21:          [unnamed]                      - 1724616215:399+16

   22:          [unnamed]                      - C

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