Operating system name and version: Ubuntu 18.04.2 LTS
Version of smsd: 3.1.21
Name and model of a modem / phone: Huawei E352s-5 (Telekom Web'n'Walk Stick Fusion III)
Provider: T-Mobile (Germany D1-Net)

It has worked for 4 years with no problems. There were no changes to the system/config.
Since last week the following error has occurred when an SMS is to be sent:

After a few attempts, the SMS will be categorized as "failed".

Do you have any idea?