This is quick fix for sentry generated backup.json file.

sentry_backup_fix.py

import json

FILENAME = "backup.json"
FILENAME_FIXED = "backup_fixed.json"


def restore():
    with open(FILENAME, "r") as f:
        lines = f.readlines()
        lines = lines[lines.index('[\n'):]
    with open(FILENAME_FIXED, "w") as f:
        f.writelines(lines)
    with open(FILENAME_FIXED, "r") as f:
        data = json.load(f)
        for d in data:
            value = d.get("fields", {}).get("value")
            if value is not None and not isinstance(value, str):
                d["fields"]["value"] = str(value)
    with open(FILENAME_FIXED, "w") as f:
        f.write(json.dumps(data, indent=4, ensure_ascii=False))
    return data


if __name__ == "__main__":
    restore()

This script caused us issues with boolean values being converted to uppercase string "False". The import didn't like them.

I've adapted the script to account for this:

import json

FILENAME = "backup.json"
FILENAME_FIXED = "backup_fixed_2.json"


def restore():
    with open(FILENAME, "r") as f:
        lines = f.readlines()
        lines = lines[lines.index('[\n'):]
    with open(FILENAME_FIXED, "w") as f:
        f.writelines(lines)
    with open(FILENAME_FIXED, "r") as f:
        data = json.load(f)
        for d in data:
            value = d.get("fields", {}).get("value")
            if isinstance(value, bool):
                d["fields"]["value"] = str(value).lower()
            elif value is not None and not isinstance(value, str):
                d["fields"]["value"] = str(value)
    with open(FILENAME_FIXED, "w") as f:
        f.write(json.dumps(data, indent=4, ensure_ascii=False))
    return data


if __name__ == "__main__":
    restore()

This script is broken and imports booleans as (encoded) strings into the database.
A True should be encoded as "gAKILg==" but gets inserted as "gAJYBAAAAFRydWVxAC4=" into the database.
After hours of debugging i finally managed to fix my installation with multiple manual db queries.
Sadly i dont have every query in my history, but some are here:

update sentry_option set value = 'gAKILg==' where value = 'gAJYBAAAAFRydWVxAC4=';
update sentry_organizationoptions set value = 'gAKILg==' where value = 'gAJYBAAAAFRydWVxAC4=';
update sentry_projectoptions set value = 'gAKILg==' where value = 'gAJYBAAAAFRydWVxAC4=';

@Lucaber , my adapted script above should be of help if you have your original exported backup.

If i remember correctly, numbers were also saved incorrectly as base64 encoded strings with quotes. 8 -> "8" | base64.
This resulted in broken "sentry:option-epoch" options.