Hi. I use several PC’s, Phones and Tablets against a remote OX-mailbox at namecheap.com. On the Android devices I have no problems. I have no problems with Windows with Thunderbird Calendar and the SoGo connector, but with eM Client, I cannot dismiss reminders. I have had this problem all the time, over a year. For me this have been a testing period for eM Client. The only solution is to open the item and remove or move the reminder and save the item again. This is no solution for recurring reminders. When i started writing this new post, I got prompted that there are 6000 similar topics!!!, and yes I read a few of them. http://imgur.com/a/GCsDd What is the fix?
I have the same problem. My solution: download the .ics file, import it to local calendar and delete the caldav account. 
Not a great solution, but it works.