Wim – I sometimes need to delete the file ~/kiwitrees/error.log if it grows too big, and yes it is recreated if new errors occur.
CAUTION – that file is NOT created by kiwitrees. It is the server logs, created by apache. You just happen to have it located, it seems, in your kiwitrees folder. Many / most people do not. It is is more commonly stored in some other location on the server.
That said, it is not a problem to delete it as it will be re-created. But your server ‘should’ also have a setting (try looking in cPanel / phpmyadmin or whatever you use) to constrain the maximum file size. Once that is reached it is zipped and stored and a new file started.