What did I forget?
I made a block that checks POP3 mail. I've made an entry of $string['pluginname'] = 'POP3' in the language files.
The pluginname is shown under Admin-plugins-blocks-manage blocks.
When I want to add the block to a page it is shown in the dropdown under "add block" as an empty line.
I'm sure I forgot something obvious.
Thanks in advance for helping me out.
Vincent