Can't change repositories in "Override the plan's default repository settings"

Summary

Unable to change the repository in Branch Plan configuration >> Source Repository >> Override the plan's default repository settings.

Environment

Bitbucket Server Linked repository

Steps to Reproduce

1. Create a branch plan
2. Go to Source Repository in the branch plan configuration
3. Enable Override the plan's default repository settings and change the repository
4. Press save
5. After saving, change to another repository
6. Press save

Expected Results

Repository changing without any issues.

Actual Results

The following error is thrown on the UI in a red banner:

Could not check if existing access key is valid: java.lang.IllegalArgumentException: Cannot read keypair, it doesn't contain any line with '-----BEGIN'

The change does not take effect because of this error

Notes

In Step 4 and 6, this error banner is also shown but does not affect the functionality:

Could not contact the Bitbucket Server instance. This repository might not function properly.

Workaround

1. Disable Override the plan's default repository settings
2. Press save
3. Enable Override the plan's default repository settings and select the new repository
4. Press save