That is one of those things that you risk when you get creative with filenames in order to make your current project go faster.
When I started programming in the seventies, I worked for a fortune 500 company and a big part of my job was helping middle managers with there new desktop computers of every type imaginable to interface with corporate databases.
It was a nightmare at times until we insisted that they never use anything but asci characters in filenames.
You just never knew when a particular existing system was going to choke on a character that worked for most systems.
Given that lxde is a lite system, I am not surprised to find that they did not include all of the overhead of coding in numerous exceptions for any and all characters that a user may decide to use.
At this point, it looks like your choices are to rethink your use of special characters in file names or to use a different tool.
I doubt that you will have any luck in convincing the authors of lite systems to start down the path of feature creep that eventually leads to bloated systems that require large fast machines that use lots of resources.
Of course, you could write your own routines to automatically escape those characters out of your file names and then put them back in again on export.
You would probably find that this would be more costly than the savings you attain by using a lite system.
I suspect that most young users and programmers will agree with you that this is a bug that ought to be fixed but you will find many of us older folks who will have no sympathy as it is a choice that you have made.
If you do a search on PCManFM special characters, you will find a lot of folks talking about it. I do not have the time to read all those articles to see if any solutions are forthcoming.
Just the fact that shell programs like the Bourne shell use the # to identify comments would be enough to keep me from ever using it in a file name.
In a brief search, I have found that a number of sites dealing with photographs and multimedia do not work with that character in a file name.
Is it really worth the risk?
Either way, I do not envy you. You have a hard decision to make.