"Access Denied" error with admin user (USR1) when checking out

After clicking the "Place your order" button on the ecommerce checkout page, I am sent to q=store/payment/trustcommerce/

However, the page displays "Access Denied", and actually displays the page twice, one on top of the other (see attached file). This does not happen with the COD payment module.

The order is recorded in the backend as "payment pending".

I'm running Drupal 5.3
PHP 4.4.7
ecommerce module 5.x-3.4
trustcommerce module 5.x-5.3.x-dev
tclink 3.4.4 (which is working, verified through another site)

I have enabled the trustcommerce module and its dependencies, and have enabled all of the permissions I can find, but either I'm missing something or there's a bug or incompatibility someplace.

Where should I look?

trace output from the trace module below:
#e3779315 T=0.000000 [REQUEST] 2007-12-04 15:53:11.499023 GET /drupal_cart/?q=store/payment/trustcommerce/10 HTTP/1.1
#e3779315 T+0.172532 [QUERY ] d=0.000299s drupal_lookup_path: SELECT COUNT(pid) FROM url_alias
#e3779315 T+0.172831 [QUERY ] d=0.002241s module_list: SELECT name, filename, throttle FROM system WHERE type = 'module' AND status = 1 ORDER BY weight ASC, filename ASC
#e3779315 T+0.175072 [QUERY ] d=0.000277s db_lock_table: LOCK TABLES variable WRITE
#e3779315 T+0.175349 [QUERY ] d=0.000536s variable_set: DELETE FROM variable WHERE name = 'dev_query'
#e3779315 T+0.175885 [QUERY ] d=0.000250s variable_set: INSERT INTO variable (name, value) VALUES ('dev_query', 'b:1;')
#e3779315 T+0.176135 [QUERY ] d=0.000214s db_unlock_tables: UNLOCK TABLES
#e3779315 T+0.176349 [QUERY ] d=0.000522s cache_clear_all: DELETE FROM cache WHERE cid = 'variables'
#e3779315 T+0.176871 [QUERY ] d=0.000919s cache_get: SELECT data, created, headers, expire FROM cache_menu WHERE cid = '1:en'
#e3779315 T+0.177790 [QUERY ] d=0.000333s ec_mail_get_name: SELECT name FROM ec_mail WHERE mid = 10
#e3779315 T+0.178123 [QUERY ] d=0.000546s store_transaction_load: SELECT * FROM ec_transaction AS st WHERE st.txnid = '10'
#e3779315 T+0.178669 [QUERY ] d=0.000565s store_transaction_load: SELECT stp.*, r.rtxnid as rtxnid, r.rnid as rnid, p.ptype FROM ec_transaction_product stp LEFT JOIN ec_renewal r ON stp.txnid = r.txnid LEFT JOIN ec_product p ON stp.vid = p.vid WHERE stp.txnid = 10
#e3779315 T+0.179234 [QUERY ] d=0.000373s store_transaction_load: SELECT stm.* FROM ec_transaction_misc stm WHERE stm.txnid = 10 ORDER BY stm.weight ASC
#e3779315 T+0.179607 [QUERY ] d=0.000372s store_transaction_load: SELECT sta.* FROM ec_transaction_address sta WHERE sta.txnid = 10
#e3779315 T+0.179979 [QUERY ] d=0.000283s ec_anon_load_product: SELECT * FROM ec_anon WHERE vid = 1
#e3779315 T+0.180262 [QUERY ] d=0.000281s trustcommerce_ec_transactionapi: SELECT * FROM ec_trustcommerce WHERE txnid = 10
#e3779315 T+0.180543 [QUERY ] d=0.000504s watchdog: INSERT INTO watchdog (uid, type, message, severity, link, location, referer, hostname, timestamp) VALUES (1, 'access denied', 'store/payment/trustcommerce/10', 1, '', 'http://1800bishop2.org/drupal_cart/?q=store/payment/trustcommerce/10', 'http://1800bishop2.org/drupal_cart/?q=cart/review', '70.244.68.130', 1196801591)
#e3779315 T+0.181047 [QUERY ] d=0.001025s list_themes: SELECT * FROM system WHERE type = 'theme'
#e3779315 T+0.182072 [QUERY ] d=0.000790s _node_types_build: SELECT nt.type, nt.* FROM node_type nt ORDER BY nt.type ASC
#e3779315 T+0.182862 [QUERY ] d=0.001661s block_list: SELECT DISTINCT b.* FROM blocks b LEFT JOIN blocks_roles r ON b.module = r.module AND b.delta = r.delta WHERE b.theme = 'garland' AND b.status = 1 AND (r.rid IN (2) OR r.rid IS NULL) ORDER BY b.region, b.weight, b.module
#e3779315 T+0.184523 [QUERY ] d=0.000478s cart_get_items: SELECT c.*, n.title, n.vid FROM node n INNER JOIN ec_cart c ON n.nid = c.nid WHERE c.cookie_id = '1'
#e3779315 T+0.185001 [QUERY ] d=0.001251s system_region_list: SELECT * FROM system WHERE type = 'theme' AND name = 'garland'
#e3779315 T+0.192527 [RESPNSE] 2007-12-04 15:53:11.681077 403 Forbidden