Open your apk with a zip tool like WinZIP or 7Zip and have a look at the "lib" folder. The subfolders in "lib" contains the native libraries and the names corresponds to the cpu type (armeabi-v7a, x86, ...).
Probably DropboxSync? It is a old lib and it could be that there is a native.so involved.
Rename Dropboxsync.jar to Dropboxsync.jar.zip and open the zip. Is there a folder with native libs? If yes; is there a folder for X86 Architecture too?
Please note that Dropboxsync is officially deprecated. It uses V1 of the API which will be removed at some point at Dropbox.
To be congrete: Timeline
June 28, 2016: API v1 is deprecated.
September 28, 2016: If you haven’t acknowledged the deprecation, new users of your app will see a warning when linking their Dropbox account.
June 28, 2017: If you haven’t acknowledged the deprecation, all users who have linked their Dropbox account to your app will receive a warning email.
September 28, 2017: API v1 endpoints are turned off.
This means that probably from Sept 29 you are not able to use DropboxSync anymore.
i had no warning msg when i used the first time dropbox on my app.
but i will check that out and if it wont work i will switch to another solution. now everything works fine.
Too much effort to check each lib. Check your apk instead, the name of the missing native lib should reasonably correspond with the java lib.
If you found a lib that exists in the lib/armeabi-v7a folder and is missing in lib/x86 then this may the reason for the error.