Struggling with migrating from Bitbucket

We want to migrate to GitLab from Bitbucket but run into problems both in terminal and SourceTree.
I know that SourceTree is not supported by GitLab but a GUI is a requirement and it is the one that we have used. If there are better alternatives please let me know. Here is the status:

Importing repos work
cloning work too but asks for the ssh passphrase repeatedly, 3 in SourceTree and 4 in terminal.
When checking out another branch in SourceTree I get

Downloading applications/synktest/assets/audio/audio_test_8sec_48khz.aac (8.4 KB)

Error downloading object: applications/synktest/assets/audio/audio_test_8sec_48khz.aac (5c89c5e): Smudge error: Error downloading applications/synktest/assets/audio/audio_test_8sec_48khz.aac (5c89c5e4c943655d25c6fd4c5701ec0b5403a3675fc895db24dccc55a0ad1a7d): batch request: git@gitlab.com: Permission denied (publickey).: exit status 255

Errors logged to /Users/DinahmoeStudio/Documents/gitlab/dmafjs-video/.git/lfs/logs/20190926T112436.219476.log

Use git lfs logs last to view the log.

error: external filter ‘git-lfs filter-process’ failed

fatal: applications/synktest/assets/audio/audio_test_8sec_48khz.aac: smudge filter lfs failed

The log doesn’t give any additional information
Checking out through terminal gives the same result

studio-dinahmoe:dmafjs-video dinahmoe$ git checkout 60fps

Downloading applications/synktest/assets/audio/audio_test_8sec_48khz.aac (8.4 KB)

Enter passphrase for key ‘/Users/DinahmoeStudio/.ssh/id_rsa’:

Error downloading object: applications/synktest/assets/audio/audio_test_8sec_48khz.aac (5c89c5e): Smudge error: Error downloading applications/synktest/assets/audio/audio_test_8sec_48khz.aac (5c89c5e4c943655d25c6fd4c5701ec0b5403a3675fc895db24dccc55a0ad1a7d): [5c89c5e4c943655d25c6fd4c5701ec0b5403a3675fc895db24dccc55a0ad1a7d] Object does not exist on the server or you don’t have permissions to access it: [404] Object does not exist on the server or you don’t have permissions to access it

Errors logged to /Users/DinahmoeStudio/Documents/gitlab/dmafjs-video/.git/lfs/logs/20190926T112839.00382.log

Use git lfs logs last to view the log.

error: external filter git-lfs smudge – %f failed 2

error: external filter git-lfs smudge – %f failed

fatal: applications/synktest/assets/audio/audio_test_8sec_48khz.aac: smudge filter lfs failed

studio-dinahmoe:dmafjs-video dinahmoe$

What do I need to do?

Anyone that can be of help?

This seems like a problem related to importing repos with LFS. This discussion seem to be relevant: https://gitlab.com/gitlab-org/gitlab-foss/issues/40401
If so, how are you supposed to migrate LFS projects from in my case Bitbucket??