This same issue was brought up earlier this year but there were no solutions. I just started getting this error about a month ago and haven't found a solution. I've tried everything.
I am using the latest version of MusicBee on Windows 10 Home 64bit, and I sign in to Windows as the Admin with Full Control of the Music directory.
I've tried running musicbee as admin.
Windows explorer says I have full access to MusicBeeLibrary.mbl
Tagging changes I make are not saved, I haven't been able to edit my library at all in the last month.
Here's my error log
12/15/2017 7:07:02 PM - D:\Music\musicbee\:Thread was being aborted.
12/15/2017 7:07:03 PM - 10.0.14393.0 - 3.1.6512.37408 - System.UnauthorizedAccessException: Access is denied. [D:\Music\musicbee\MusicBeeLibrary.mbl]
at #=q93wXi7CDpzzywjSsIbmQ$A==.#=qByNImwbtoLFE2Kjy8lrwEA==.#=qxg9TWWPrADk3nnZm54ibvg==.#=q6EHOl4mbJBYO7rhFu_YP$A==(#=quUsQu7wW7Q8y0$dMYNdliw== #=q8RpexOdQm4ulKun26sj_hQ==, #=quUsQu7wW7Q8y0$dMYNdliw== #=qX3K5prp2xbSAqqShO09G6A==, Boolean #=qlhX6gr7$_f$lu$DKGUOJ$Q==)
at #=qjBybNsMAn5ktqthFmE5Rdsm6sz3u$HpNLSwEbt$WoqI=.#=qgqgORhfEPGYwbMdKpZ3lZ0F5mW467Do5HUiQVefqRVE=.#=qCw93eIuBKv5JNaH1gf0FMg==()
at #=qjBybNsMAn5ktqthFmE5Rdsm6sz3u$HpNLSwEbt$WoqI=.#=qgqgORhfEPGYwbMdKpZ3lZ0F5mW467Do5HUiQVefqRVE=.#=q6KHVV8wg3ddt3ut40J4ahg==()
at MusicBee.Media.Library.#=qcT0YBgivSNHyvM7N5d9CJQ==()
at MusicBee.MusicBeeApplication.#=qNvhPIG$qugV8pM_5G0nYaA==.#=qOXHHeCRukxi9lmprOoCxqQ==(CloseReason #=qopsY9MTm421QcuIH4PrPLw==)