First I prepared some database init parameters in a document (actually I reused some old documentation I had done in a previous project).
The "alter system ..." command worked like a charm.
During the preparations I needed to restart the database instance. To my surprise I received an error that there was an issue with an init parameter.
ORA-16024: parameter LOG_ARCHIVE_DEST_1 cannot be parsed
Google did not really help me. So I decided to create a pfile from the spfile and had a look into it.
There it became evident what the error was. Somehow during the copy & paste the end-of-lines were copied as well - leaving my init parameter with some newlines in it.
Made one line from it in the pfile, started the db and created a new spfile. Now it worked like a charm.