Backup chain appears to be wrong, transaction log is not flushed after backup (SQL Server 2019)

Since this week, I have been getting an error message from SQL Server when backing up the logs:

Msg 3052, Level 16, State 1, Line 13 The backup log failed to record updates to database ‘DB01’. Subsequent log backups will be required to advance the savepoint from (1273423:2289656:216) to (1273436:1546104:1) once log space is made available for their logging.

I tried to find specific information regarding this error, but without success. After looking at the save game information, I can see the problem (first_lsn does not change):

| database_name   | backup_start_date       | first_lsn              | last_lsn               | database_backup_lsn    | checkpoint_lsn         | BackupType      | recovery_model |  
| --------------- | ----------------------- | ---------------------- | ---------------------- | ---------------------- | ---------------------- | --------------- | -------------- |  
| DB01            | 2024-07-26 08:05:11.000 | 1273423000228965600216 | 1273433000536109600001 | 1273423000228965600216 | 1273433000536041600001 | Transaction Log | FULL           |  
| DB01            | 2024-07-26 07:56:36.000 | 1273423000228965600216 | 1273433000526588000001 | 1273423000228965600216 | 1273433000526485600119 | Transaction Log | FULL           |  
| DB01            | 2024-07-26 07:52:42.000 | 1273423000228965600216 | 1273433000524165600001 | 1273423000228965600216 | 1273433000524094400001 | Transaction Log | FULL           |  
| DB01            | 2024-07-26 07:35:21.000 | 1273423000228965600216 | 1273433000509740800001 | 1273423000228965600216 | 1273433000509678400001 | Transaction Log | FULL           |  
| DB01            | 2024-07-26 07:05:11.000 | 1273423000228965600216 | 1273433000485428800001 | 1273423000228965600216 | 1273433000485365600001 | Transaction Log | FULL           |  
| DB01            | 2024-07-26 06:35:10.000 | 1273423000228965600216 | 1273433000465804000001 | 1273423000228965600216 | 1273433000465774400001 | Transaction Log | FULL           |  
| DB01            | 2024-07-26 06:11:54.000 | 1273423000228965600216 | 1273433000453656800001 | 1273423000228965600216 | 1273433000453617600001 | Transaction Log | FULL           |  
| DB01            | 2024-07-26 01:00:40.000 | 1273423000228965600216 | 1273423000229044800001 | 1273423000154140800048 | 1273423000228965600216 | Full                | FULL           |  
| DB01            | 2024-07-26 01:00:40.000 | 1273423000228965600216 | 1273423000229044800001 | 1273423000154140800048 | 1273423000228965600216 | Full                | FULL           |  
| DB01            | 2024-07-26 01:00:40.000 | 1273423000228965600216 | 1273423000229044800001 | 1273423000154140800048 | 1273423000228965600216 | Full                | FULL           |  
| DB01            | 2024-07-26 01:00:40.000 | 1273423000228965600216 | 1273423000229044800001 | 1273423000154140800048 | 1273423000228965600216 | Full                | FULL           |  

However, I have no idea how to fix this. I tried going into simple recovery mode and rolling back, followed by a new full backup, but that didn’t change anything. I also increased the log file size a bit, but that didn’t help either.

I would really appreciate any help on this.

Best, Ju