as I dont have mysql 5 installed, do you happen to have the error message it generates handy?

I hope i'm getting you right and You're asking for error message, so given hereunder:

APPLICATION ERROR 401
Database query failed. Error received from database was #1054: Unknown column 'p.field_id' in 'on clause' for the query: SELECT f.name, f.type, f.access_level_r, f.default_value, f.type, s.value
FROM mantis_custom_field_project_table AS p, mantis_custom_field_table AS f
LEFT JOIN mantis_custom_field_string_table AS s
ON p.field_id=s.field_id AND s.bug_id='35'
WHERE p.project_id = '76' AND p.field_id = f.id
ORDER BY p.sequence ASC, f.name ASC

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I have MySQL 5.0.18 on production server but on my local mashine installed v.4.0.+ so when I tried to run the the query via phpAdmin on v5.0.18 it triggers error while on my local mashine with v4.0.+ it works fine.
Like I said in Add-Info, the orignal query structure seems to be lousy:
FROM clause defines 2 tables, then one explicit JOIN clause and then one more implicit JOIN in WHERE clause + constraint, not the best query composing style.
Better is:
FROM clause defines a major select table
JOIN block with all additional tables you need
and then in WHERE clause are coming all the constraints U get based on cartesian product table U get as result of defined JOINs.

Neverless that's not the question of style but functionality.

I think that this code was re-writted in the RCs leading up to 1.0.0. Could you upgrade and retest?

Unfortunately I can't upgrade and re-test it. I'm pretty much in final stages to launch my production server. The whole thing rised up while switching Mantis from local mashine configured with MySql v4.0.+ to production server with MySQL 5.0.18. Right now I'm fully syncronized between local mashine and production one, so I don't even have older MySQL v.4 installed and available.
I fixed the problem changing the query structure like I mentioned in my note but if You guys already took care of that then ... np.
I just thought that it might be helpful.

It was fixed in 1.0.0a1; see 0006394.