That error appears to have cleared itself now.
Back on topic, I have tried exporting a manual playlist and can confirm that tracks on the same drive as the playlist have a relative path, whilst tracks on a different drive have an absolute path. See below for an example
#EXTM3U
#EXTINF:115,The Alan Parsons Project - Sirius (Instrumental)
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\01 - Sirius (Instrumental).mp3
#EXTINF:276,The Alan Parsons Project - Eye In The Sky
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\02 - Eye In The Sky.mp3
#EXTINF:291,The Alan Parsons Project - Children Of The Moon
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\03 - Children Of The Moon.mp3
#EXTINF:130,The Alan Parsons Project - Gemini
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\04 - Gemini.mp3
#EXTINF:441,The Alan Parsons Project - Silence And I
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\05 - Silence And I.mp3
#EXTINF:261,The Alan Parsons Project - You're Gonna Get Your Fingers Burned
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\06 - You're Gonna Get Your Fingers Burned.mp3
#EXTINF:292,The Alan Parsons Project - Psychobabble
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\07 - Psychobabble.mp3
#EXTINF:214,The Alan Parsons Project - Mammagamma
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\08 - Mammagamma.mp3
#EXTINF:234,The Alan Parsons Project - Step By Step
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\09 - Step By Step.mp3
#EXTINF:305,The Alan Parsons Project - Old And Wise
H:\The Alan Parsons Project\Eye In The Sky (Remastered)\10 - Old And Wise.mp3
#EXTINF:285,Alanis Morissette - All I Really Want
\Alanis Morissette\Jagged Little Pill\01 - All I Really Want.mp3
#EXTINF:249,Alanis Morissette - You Oughta Know
\Alanis Morissette\Jagged Little Pill\02 - You Oughta Know.mp3
#EXTINF:188,Alanis Morissette - Perfect
\Alanis Morissette\Jagged Little Pill\03 - Perfect.mp3
#EXTINF:221,Alanis Morissette - Hand In My Pocket
\Alanis Morissette\Jagged Little Pill\04 - Hand In My Pocket.mp3
#EXTINF:176,Alanis Morissette - Right Through You
\Alanis Morissette\Jagged Little Pill\05 - Right Through You.mp3
#EXTINF:300,Alanis Morissette - Forgiven
\Alanis Morissette\Jagged Little Pill\06 - Forgiven.mp3
#EXTINF:239,Alanis Morissette - You Learn
\Alanis Morissette\Jagged Little Pill\07 - You Learn.mp3
#EXTINF:267,Alanis Morissette - Head Over Feet
\Alanis Morissette\Jagged Little Pill\08 - Head Over Feet.mp3
#EXTINF:280,Alanis Morissette - Mary Jane
\Alanis Morissette\Jagged Little Pill\09 - Mary Jane.mp3
#EXTINF:229,Alanis Morissette - Ironic
\Alanis Morissette\Jagged Little Pill\10 - Ironic.mp3
#EXTINF:227,Alanis Morissette - Not The Doctor
\Alanis Morissette\Jagged Little Pill\11 - Not The Doctor.mp3
#EXTINF:293,Alanis Morissette - Wake Up
\Alanis Morissette\Jagged Little Pill\12 - Wake Up.mp3
#EXTINF:491,Alanis Morissette - You Oughta Know [Alternate Take]
\Alanis Morissette\Jagged Little Pill\13 - You Oughta Know [Alternate Take].mp3