I ran across this Ringo Starr & His All-Starr Band fan site with some info (it's French, which is why I had to clean up the grammar):
http://www.victorbaissait.fr/RingoSt.../Tour/1997.php
"Dave Mason [was] fired four days before the start of the [1997] tour, was not replaced."
Notes (text in double quote marks is the site author's, text in single quote marks within are from the band members; exxception being Simon Kirke's quote, which though in double quote marks is a quote directly from him):
Per multi-instrumentlist Mark Rivera: "He was fired as he didn't learn his part. '
Never once came prepared... and anytime things were right he'd puff his chest up like he did everything.' He was fired after Ringo asked the band if he should be fired."
Per drummer Simon Kirke: "You can read between the lines there were musical differences and he didn’t make the final lineup."
Per Peter Frampton in his book: "'
Dave couldn’t seem to learn our songs, but at rehearsals, every time we made a mistake on one of his, he would give whoever the stink eye.' (Page 190). [He also] explained [Mason skipped] the last [rehearsal] run-through [and] told Ringo that he had a doctor’s appointment [which] turn[ed out] to be false."
In the All-Starr Band, half the material is Ringo's and half consists of 2-3 songs from most (and for one tour all) of the other members. So Mason would have been expected to learn not just Ringo's material, but the material of the other band members as well.
Per guitarist Peter Frampton, he didn't bother to learn the others' material (yet got upset when they made a mistake on his). Then he skipped the last rehearsal with a phony reason and was fired four days before the tour.
Per multi-instrumentalist Mark Rivera, Mason "never once came prepared," had an ego problem, and was fired after Ringo consulted with the rest of the band.
As Mason wasn't replaced (I imagine there was too little time), the 1997-98 All-Starr Band lineup had only six members (seven would become the norm) and was the first with a single guitarist. Mason's firing set the single-guitar template for all but one of the next few tours, before Ringo returned to a two-guitar lineup in 2006.
Sounds like Mason isn't much of a band guy; which would explain why he was in and out of Traffic 1967-71, was in and out of Delaney & Bonnie even within a single tour, was out of Derek & The Dominos after a single gig, was put to the side while a guitarist from outside the band played on all of Christine's songs on
Time, did not participate in Traffic's 1994 reunion album or tour (the latter conflicted with Fleetwood Mac, FWIW), was fired from Ringo's band in 1997, and (though he was present for the group's induction) did not perform with Traffic at the 2004 Rock And Roll Hall Of Fame ceremony.