Ok, I realize this is very vague, so before posting this issue I did some research myself:
Problem 1:
About 10-20 Times a day "Wunderlist problem 404" is shown in the error-log. Some further analysis showed that in my case this always comes from node_helper.js when getUsers() is invoked and this invoke fails. This seems to be transient, the system just continues to issue the same call again after "interval" seconds (120 in my config) and all is OK again So these error log entries seem to indicate some transient problems in the Wunderlist API calls that is of no visible consequence in functionality for me.
Problem 2:
about 1 - 2 times a day a "Wunderlist 500 unknown error" appears in the log. After two of these errors (I have two lists shown on the display) the module stops executing, no more activity and updates. Further analysis shows this error log entry comes from fetcher.js where the xxx.tasks.forList(ListID) function can succeed and set the timer again for the next call. If that one fails (which it does sporadically, also here it seems that the Wunderlist API has a habit of returning an error now and then), the function puts an entry to the error log, ends there and does not start the timer again. After both lists failed, the fetchers stop fetching completely. I temporarily repaired it by starting the timer again also on the error branch (limited to 10 times by a retry-counter). Let's see if it works for me. I just did it, and I have to wait for a day or so to see if it works.
Ok, I realize this is very vague, so before posting this issue I did some research myself:
Problem 1:
Problem 2: