Since moving to Windows 10, I have been experiencing intermittent issues with modifying playlists. Right now, one playlist is being particularly stubborn and I can't add anything to it at all. Here is the error output (with the playlist file path edited for privacy).
MusicBee v3.3.7052.32107 (Win10.0), 26 Apr 2019 16:31:
#=zCm_jlXqojqin4hmN2$rqgIw=+#=zhdGcoCjemWO7+#=zr5Xu6YrAO7PJ: The process cannot access the file because it is being used by another process. [*folder path*\playlist.m3u]
at #=zCm_jlXqojqin4hmN2$rqgIw=.#=zhdGcoCjemWO7.#=zCMpeRYQ=.#=zhMRCW9dmtpJS(#=zBcACJC4dkMpBxZUcZg== #=z5iurmoM=, FileMode #=zUuH_rKw=, FileAccess #=zlVFc1$s=, FileShare #=zHuGCNBY=, Int32 #=zW_3FDsE=, Boolean #=zCBbpa99s_XsE)
at #=z178JtPIbn4KK6ndUk1Il$bDsFbDj.#=zhMRCW9dmtpJS(#=zBcACJC4dkMpBxZUcZg== #=z5iurmoM=, FileMode #=zUuH_rKw=, FileAccess #=zlVFc1$s=, Int32 #=zW_3FDsE=)
at #=zGZR0Nrig_6GVS4YAlgjjFcw=.#=z$YcPUv0=(List`1 #=zWJmDr50=)
at #=zGZR0Nrig_6GVS4YAlgjjFcw=.#=zPLRto5A=(IList`1 #=zWJmDr50=)
at #=zc7Dq5voZbtbNY3cZzd_1k3t_Onmm.#=zdhr3ASVPJA6m(#=zulo0opTWZnuTpvvKd3kRzI4= #=z4oIkusA=, List`1 #=zWJmDr50=, List`1 #=zI50Kx0qRG9e$)
at #=zc7Dq5voZbtbNY3cZzd_1k3t_Onmm.#=zSvZHigcOun0y.#=qHIOJlFo2DGgK1oJJK_y7V5YtUfbtkYEUtxxaBmSEllE=._Lambda$__0(List`1 #=zEu_A1caAfFcY)