Hello Jaromír,
Greetings! Welcome to Microsoft Q&A Platform.
I understand that now your storage account is in migration phase and hence it is showing the message "Management operations are not allowed" and that goes away when the migration is fully completed and there is no particular time frame for migration completion and mostly it depends on the size of data in storage account and network bandwidth. But you'll be able to perform control plane tasks on the storage account. You should be able to access its content (on the data plane) in the meantime.
Once you still see the same message after few hours, please select the examine status and look for the migration status and if it is successfully migrated then ensure that all necessary data has been successfully transferred to the new storage account and confirm that no active operations are pending (such as uploads, downloads, or other management operations).
If migration was aborted or stuck, then please retry.
If possible, you may try to migrate Classic Storage account to ARM using PowerShell or CLI and check if that helps.
Step-by-Step process: https://learn.microsoft.com/en-us/azure/storage/common/classic-account-migrate?tabs=azure-powershell
You may also refer to the suggestion mentioned in this article: https://learn.microsoft.com/en-us/azure/virtual-machines/migration-classic-resource-manager-errors
refer for more details on migration- https://learn.microsoft.com/en-us/azure/storage/common/classic-account-migrate?tabs=azure-portal, https://learn.microsoft.com/en-us/azure/storage/blobs/upgrade-to-data-lake-storage-gen2-how-to?tabs=azure-portal, https://learn.microsoft.com/en-us/azure/storage/blobs/data-lake-storage-migrate-gen1-to-gen2-azure-portal, https://stackoverflow.com/questions/76706200/azure-storage-account-stuck-on-0-for-data-lake-gen2-validation.
Hope this answer helps! Please let us know if you have any further queries. I’m happy to assist you further.
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