I did an upgrade of Zabbix 1.8 to 2.0 and after the upgrade, my web scenario's no longer fire. The last check field remains at the time before the upgrade and newly added scenarios just say: Never.

Interval is set at 5 minutes. The server log says http_pollers are present:

9108:20120615:105446.389 Starting Zabbix Server. Zabbix 2.0.0 (revision 27675).
9108:20120615:105446.389 ****** Enabled features ******
9108:20120615:105446.389 SNMP monitoring: YES
9108:20120615:105446.389 IPMI monitoring: NO
9108:20120615:105446.389 WEB monitoring: YES
9108:20120615:105446.389 Jabber notifications: YES
9108:20120615:105446.389 Ez Texting notifications: YES
9108:20120615:105446.389 ODBC: NO
9108:20120615:105446.389 SSH2 support: NO
9108:20120615:105446.389 IPv6 support: NO
9108:20120615:105446.389 ******************************
9111:20120615:105446.630 server #2 started db watchdog #1
9110:20120615:105446.631 server #1 started configuration syncer #1
9123:20120615:105446.639 server #14 started icmp pinger #1
9121:20120615:105446.645 server #12 started trapper #4
9120:20120615:105446.645 server #11 started trapper #3
9124:20120615:105446.645 server #15 started alerter #1
9119:20120615:105446.646 server #10 started trapper #2
9118:20120615:105446.646 server #9 started trapper #1
9125:20120615:105446.647 server #16 started housekeeper #1
9125:20120615:105446.647 executing housekeeper
9126:20120615:105446.648 server #17 started timer #1
9122:20120615:105446.640 server #13 started trapper #5
9127:20120615:105446.651 server #18 started http poller #1
9128:20120615:105446.651 server #19 started http poller #2
9108:20120615:105446.653 server #0 started [main process]
9133:20120615:105446.653 server #24 started history syncer #1
9134:20120615:105446.653 server #25 started history syncer #2
9135:20120615:105446.654 server #26 started history syncer #3
9131:20120615:105446.654 server #22 started http poller #5
9136:20120615:105446.655 server #27 started history syncer #4
9137:20120615:105446.655 server #28 started escalator #1
9130:20120615:105446.655 server #21 started http poller #4
9129:20120615:105446.655 server #20 started http poller #3
9138:20120615:105446.656 server #29 started proxy poller #1
9139:20120615:105446.656 server #30 started self-monitoring #1
9115:20120615:105446.715 server #6 started poller #4
9113:20120615:105446.715 server #4 started poller #2
9114:20120615:105446.716 server #5 started poller #3
9117:20120615:105446.716 server #8 started unreachable poller #1
9116:20120615:105446.716 server #7 started poller #5