Hello,
I assume that's because you wrote the base64 value to the entry, what you want to store in the database is the raw secret (output of base32 -d). But if you want to use ldapmodify, you need to use LDIF and that's where you use base64 and "::" to indicate the data you provided is base64 encoded, that's what's described in the guide.
Thank you, it's working now. Probably, the missing second ':' was the problem - I had it in my ldif in the beginning, but at some point removed it because I thought it was a typo ...
Regards Christoph