SQL Server: get meta-Info about LDF-File
This returns a String like '00000064:000000e9:0033'. But how can I convert this into a timestamp?


You can't.  An LSN is not a timestamp, nor does it store any portion of date and/or time in it's bits.

SQL Server does not log the datetime stamp for every logged operation.  It does keep track of these timestamps for transactions, but not for each and every single logged operation.

Note: You are trying to reference a Change Data Capture DMF with sys.fn_cdc_map_lsn_to_time.

I think you are taking the wrong approach to some sort of question that you are trying to answer.  Datetime stamps on transaction log records wouldn't really tell you much, even if it is possible.

Let's take a step back and let us know why you're trying to do this.