What opens is one with nothing in it, no custom views, nothing, like the one you get when you first install MC after purchasing it.
If it is a completely empty Library, then it sounds like something is deleting the Library when you do a restart. MC will recreate the Default Library when it starts if it doesn't exist, so if it ever gets deleted then MC can still run. MC won't run without a Library to open.
I suggest that you check the Default Library directory in "C:\Users\[UserID]\AppData\Roaming\J River\Media Center 26\Library" before restarting MC. Specifically note the size of the directory and the latest "Date modified" including the time of the files in there. Then do a Library Backup and restart MC as you normally do. Then check the Default Library directory again. Does it change size, becoming smaller? Do the most recent file times change to when you restarted MC? Basically, have the real Library files been deleted and replaced with a Default set?
While resolving this, as Wer says, do lots more Library Backups. In fact, do one immediately before each restart. You shouldn't lose anything.
I see you have three other DLNA Servers running on your network, so you do have a busy environment. But I don't think they would be contributing to this issue, as long as the MC Library is set up correctly. Frankly, I can't think what would be deleting the MC Library just because of a restart, unless the restart process you are using includes something that explicitly deletes the Library, perhaps in a modified MC shortcut?
Also, I thought the "slow down over time" issue that creates the need to restart had been resolved, or at least reduced significantly. You do have a large Library though, at 260,000 files. The slow down you see also hints at something else going on, perhaps interfering with MC writing Library files or tags, or analysing files. You may have to do some serious investigation to work our what is affecting MC.