How to Fix log_reuse_wait_desc=LOG_BACKUP? LOG Backup not possible
Now that you've changed the recovery model on the database from full to simple and back to full again, the database is running in 'pseudo-simple' mode. Taking a database backup is required now before a log backup will do any meaningful work. See here for some further details. To summarize:


In order for a log backup to be useful for a restore, there needs to be a database backup along with log backups that cover all the LSNs since the time of that database backup.
When you change the recovery model to simple, SQL Server breaks the log backup chain. Then when you change it back to full, the database continues to act like it is in simple mode until someone creates a new log backup chain by taking a database backup. This is known as pseudo-simple mode. The reason this is implemented this way is to prevent someone from taking log backups that could never be used for recovery because they aren't part of a log backup chain that begins with a database backup.
Once a database backup is taken, a new log backup chain begins and the database begins functioning in the full recovery model. At this point your log backups should work, or at least fail for another reason with a more descriptive error.


This will be at least one reason why your log backups aren't functioning now.