I can give you one bit of advice that may help immediately: Please check to see that you have the existing file structure (you can see this in your file browser) that matches what is set in the preferences in Seamly2D.
Please make sure that the folders exist and that your pattern .val files are in the directory indicated by “My Patterns” and the .vit files are in the directory indicated by “My Individual Measurements” and the .vst files are in the directory indicated by “My Multisize Measurements”.
I have seen problems if those criteria are not met and that could be what you are seeing.
I have several versions of Ubuntu installed and I have not seen this particular problem. I have seen some flakey things that are related to Qt, which is a commercial development platform that is part of what is supposed to make the software run on different operating systems. I will look into possible compatibility issues with Linux Mint 18.3 Cinnamon 64 bit. I suspect that it is an incompatibility between Qt5.5.1 and your particular flavor and version of Linux.
I am so sorry that you are having this problem, and I also apologise in advance if I am not able to help you immediately. I am about to be on a trip with only sporadic internet access for 45 days.
I will do what I can this evening. Could you please post information like I show here from my system.
Specifically I need to know what information you can provide about your kernel version of linux, the Qt version that is loaded by your operating system, and desktop information. As you can see on my screenshot, the system I am using has KDE Plasma version 5.12.7 and KDE Frameworks Version 5.44.0 and Qt version 5.9.5 and Linux kernel version 4.15.0-44 (64 bit)
I have seen problems with opening files such as you describe on some, but not all of the various flavors of Ubuntu I have tested.