Files are "stuck" in the upload queue. The files exist, but when retrying the App reports a "connection error". From the device log:
~~~
03-04 12:26:03.086 7438 7438 D UploadsStorageManager: updateUpload returns with: 1 for file: /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg
03-04 12:26:03.086 7438 7438 D UploadsStorageManager: notifyObserversNow
03-04 12:26:03.101 7438 8099 V UploadsStorageManager: Updating /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg with status:UPLOAD_IN_PROGRESS and result:UNKNOWN (old:/storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg status:UPLOAD_IN_PROGRESS result:-1)
03-04 12:26:03.101 7438 8099 V UploadsStorageManager: Updating /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg with status=UPLOAD_IN_PROGRESS
03-04 12:26:03.115 7438 8099 D UploadsStorageManager: updateUpload returns with: 1 for file: /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg
03-04 12:26:03.115 7438 8099 D UploadsStorageManager: notifyObserversNow
03-04 12:26:03.138 7438 8099 V UploadsStorageManager: Updating /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg with status=UPLOAD_IN_PROGRESS
03-04 12:26:03.144 7438 8099 D UploadsStorageManager: updateUpload returns with: 1 for file: /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg
03-04 12:26:03.144 7438 8099 D UploadsStorageManager: notifyObserversNow
03-04 12:26:03.179 7438 7438 D ExpandableUploadListAdapter: refreshView
03-04 12:26:03.179 7438 7438 D ExpandableUploadListAdapter: loadUploadItemsFromDb
03-04 12:26:03.181 7438 8099 E FileUploader: Error uploading
03-04 12:26:03.181 7438 8099 E FileUploader: java.lang.NullPointerException: Attempt to invoke virtual method 'long com.owncloud.android.datamodel.OCFile.getFileId()' on a null object reference
03-04 12:26:03.181 7438 8099 E FileUploader: at com.owncloud.android.operations.UploadFileOperation.run(UploadFileOperation.java:374)
03-04 12:26:03.181 7438 8099 E FileUploader: at com.owncloud.android.lib.common.operations.RemoteOperation.execute(RemoteOperation.java:145)
03-04 12:26:03.181 7438 8099 E FileUploader: at com.owncloud.android.lib.common.operations.RemoteOperation.execute(RemoteOperation.java:149)
03-04 12:26:03.181 7438 8099 E FileUploader: at com.owncloud.android.operations.common.SyncOperation.execute(SyncOperation.java:93)
03-04 12:26:03.181 7438 8099 E FileUploader: at com.owncloud.android.files.services.FileUploader.uploadFile(FileUploader.java:1090)
03-04 12:26:03.181 7438 8099 E FileUploader: at com.owncloud.android.files.services.FileUploader$ServiceHandler.handleMessage(FileUploader.java:1023)
03-04 12:26:03.181 7438 8099 E FileUploader: at android.os.Handler.dispatchMessage(Handler.java:102)
03-04 12:26:03.181 7438 8099 E FileUploader: at android.os.Looper.loop(Looper.java:148)
03-04 12:26:03.181 7438 8099 E FileUploader: at android.os.HandlerThread.run(HandlerThread.java:61)
03-04 12:26:03.181 7438 8099 D UploadsStorageManager: updateDatabaseUploadResult uploadResult: com.owncloud.android.lib.common.operations.RemoteOperationResult@d7fbf32 upload: com.owncloud.android.operations.UploadFileOperation@e51d483
03-04 12:26:03.187 7438 8099 V UploadsStorageManager: Updating /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg with status:UPLOAD_FAILED and result:NETWORK_CONNECTION (old:/storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg status:UPLOAD_IN_PROGRESS result:-1)
03-04 12:26:03.187 7438 8099 V UploadsStorageManager: Updating /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg with status=UPLOAD_FAILED
03-04 12:26:03.192 7438 8099 D UploadsStorageManager: updateUpload returns with: 1 for file: /storage/emulated/0/DCIM/Camera/IMG_20180301_162013346.jpg
03-04 12:26:03.192 7438 8099 D UploadsStorageManager: notifyObserversNow
03-04 12:26:03.193 7438 8099 D FileUploader: NotifyUploadResult with resultCode: HOST_NOT_AVAILABLE
03-04 12:26:03.197 7438 8099 D FileUploader: Stopping command after id 114
~
Manually uploading the same file works.

Files in the auto-upload queue should be uploaded.
Unclear. Apparently it due to the target folder not existing (see comment by @TeaspoonDD below).
Android version: 6.0.1
Device model: Moto X Play
Stock or customized system: Stock
Nextcloud app version: 3.0.2
Nextcloud server version: 13.0.0
Same issue here. OnePlus5 Nextcloud 13.0.0 server, 3.0.2 app.
Had the same issue. Samsung A5 2017 / Android 7.0 / App 3.0.2 / nextcloudpi v0.46.34 with Nextcloud 12.0.4
Made the month folder "03" in "SofortUpload" manually. The next picture auto upload worked fine.
Can confirm the workaround by @TeaspoonDD: After creating SofortUpload/2018/03 (but not SofortUpload/{Snapseed, WhatsApp Images}/2018/03 the two images that are supposed to end up in SofortUpload/2018/03 went through and the others did not.
Confirmed; manually creating the monthly directory allows the uploads to proceed.
I'm having the same problem. I manually created the missing (monthly) directories, and then had to manually navigate and view the created directories in the app before the auto uploads started working again. I had manually created the directories in the nextcloud web interface, which alone doesn't fix the problem.
Same issue here on SGS8. Created folder manually in web interface (logging into console and creating the directory didn't work) and then visited the folder on on the phone.
Woth mentioning that the iphone app worked fine. It created the folder and everything
I have the same issue. @AndyScherzinger @mario @tobiasKaminsky I suppose this will be fixed in 3.0.3?
Hopefully yes @enoch85 :)
Would like to confirm and show what worked for me:
1: Manually create 03 folder
2: Navigate there in app
3: Take new picture
After step 3 it started to upload all other photo's aswell :)
@wouterverduin @tribut try out latest 3.0.3
This is fixed in 3.0.3 just tested myself.
Please reopen if you think I'm wrong.
Most helpful comment
Hopefully yes @enoch85 :)