Only allow 1800 loops in a thread, to prevent email IMAP fetch stops working if mailserver is once unreachable. Fixes issue#1861.

app/models/scheduler.rb

@@ -233,7 +233,9 @@ class Scheduler < ApplicationModel
 
       # start loop for periods equal or under 5 minutes
       if job.period && job.period <= 5.minutes
+        loop_count = 0
         loop do
+          loop_count += 1
           _start_job(job)
           job = Scheduler.lookup(id: job.id)
 
@@ -246,6 +248,9 @@ class Scheduler < ApplicationModel
           # exit if there is no loop period defined
           break if !job.period
 
+          # only do a certain amount of loops in this thread
+          break if loop_count == 1800
+
           # wait until next run
           sleep job.period
         end