Following error occurs during the verification step of trailhead project "Create a satisfaction survey", step 3 "Add a screen for thanking the customer" : error while verifying this challenge, reference error id: EECQWTHH

Suddenly get above error when clicking custom button "Add Coverages" on page https://tomtom--config.cs3.my.salesforce.com/a1IQ0000002zpKY, which calls a custom page. 
This has been working before.

An error has occurred while processing your request. The salesforce.com support team has been notified of the problem. If you believe you have additional information that may be of help in reproducing or correcting the error, please contact Salesforce Support. Please indicate the URL of the page you were requesting, any error id shown on this page as well as any other related information. We apologize for the inconvenience. 

Thank you again for your patience and assistance. And thanks for using salesforce.com! 

Error ID: 1427357906-6440 (1257850963)
 

Trying to query for all opportunity product line dates and child schedule dates :
Any idea why following query does not work ?
"select servicedate, (select scheduledate FROM opportunitylineitemschedule) from opportunitylineitems limit 20"
(ERROR at Row:1:Column:81 sObject type 'opportunitylineitems' is not supported.)
It does work from opportunity to opportunitylineitem
"select closedate, (select servicedate FROM opportunitylineitems) from opportunity limit 20"

Following error occurs during the verification step of trailhead project "Create a satisfaction survey", step 3 "Add a screen for thanking the customer" : error while verifying this challenge, reference error id: EECQWTHH

Suddenly get above error when clicking custom button "Add Coverages" on page https://tomtom--config.cs3.my.salesforce.com/a1IQ0000002zpKY, which calls a custom page. 
This has been working before.

An error has occurred while processing your request. The salesforce.com support team has been notified of the problem. If you believe you have additional information that may be of help in reproducing or correcting the error, please contact Salesforce Support. Please indicate the URL of the page you were requesting, any error id shown on this page as well as any other related information. We apologize for the inconvenience. 

Thank you again for your patience and assistance. And thanks for using salesforce.com! 

Error ID: 1427357906-6440 (1257850963)
 

Trying to query for all opportunity product line dates and child schedule dates :
Any idea why following query does not work ?
"select servicedate, (select scheduledate FROM opportunitylineitemschedule) from opportunitylineitems limit 20"
(ERROR at Row:1:Column:81 sObject type 'opportunitylineitems' is not supported.)
It does work from opportunity to opportunitylineitem
"select closedate, (select servicedate FROM opportunitylineitems) from opportunity limit 20"