Suddenly Auth.Net rejects my expiry info

Question

I set up my form using the full Authorize.net "Plug-In" several months ago using the "unauthorized guide" and everything has been working smoothly.
But Tuesday we got a donation that showed no expiration month or year on the email to the webmaster - just blanks, no zeros or numbers.
I contacted the donor and asked for the missing information.
I took it and tried to submit the form myself. I put in the expiration info, and got this error message:


x_login=6zxxxxxxxxxxx&x_version=3.1&x_delim_char=%7C&x_delim_data=TRUE&x_url=FALSE&x_type=AUTH_CAPTURE&x_method=CC&x_tran_key=6Xxxxxxxxxxxxx&x_relay_response=FALSE&x_card_num=4xxxxxxxxxxxxxx9&x_exp_date=&x_description=&x_first_name=Kxxxxr&x_last_name=Zxxxxxr&x_amount=100&x_address=3xx0+Sxxh+Sxxe+Drive&x_city=Chicago&x_state=IL&x_zip=6xxx3&x_invoice_num=&x_cust_id=&x_company=&x_country=United+States&x_phone=7xxxxxxxx5&x_fax=&x_email=kxxxxxxxxm&
Response Code: 	Error
Response Subcode: 	2
Response Reason Code: 	33
Response Reason Text: 	Expiration date is required.
Approval Code: 	NO VALUE RETURNED
AVS Result Code: 	P
Transaction ID: 	0
Invoice Number (x_invoice_num): 	NO VALUE RETURNED
Description (x_description): 	NO VALUE RETURNED
Amount (x_amount): 	100.00
Method (x_method): 	CC
Transaction Type (x_type): 	auth_capture
Customer ID (x_cust_id): 	NO VALUE RETURNED
Cardholder First Name (x_first_name): 	Kxxxxxr
Cardholder Last Name (x_last_name): 	Zxxxxxy
Company (x_company): 	NO VALUE RETURNED
Billing Address (x_address): 	3xxxxxxxve
City (x_city): 	Chicago
State (x_state): 	IL
ZIP (x_zip): 	6xxx3
Country (x_country): 	United States
Phone (x_phone): 	7xxxxxxxxxx5
Fax (x_fax): 	NO VALUE RETURNED
E-Mail Address (x_email): 	kxxxxxxxxxxxxxxxm
Ship to First Name (x_ship_to_first_name): 	NO VALUE RETURNED
Ship to Last Name (x_ship_to_last_name): 	NO VALUE RETURNED
Ship to Company (x_ship_to_company): 	NO VALUE RETURNED
Ship to Address (x_ship_to_address): 	NO VALUE RETURNED
Ship to City (x_ship_to_city): 	NO VALUE RETURNED
Ship to State (x_ship_to_state): 	NO VALUE RETURNED
Ship to ZIP (x_ship_to_zip): 	NO VALUE RETURNED
Ship to Country (x_ship_to_country): 	NO VALUE RETURNED
Tax Amount (x_tax): 	NO VALUE RETURNED
Duty Amount (x_duty): 	NO VALUE RETURNED
Freight Amount (x_freight): 	NO VALUE RETURNED
Tax Exempt Flag (x_tax_exempt): 	NO VALUE RETURNED
PO Number (x_po_num): 	NO VALUE RETURNED
MD5 Hash: 	82xxxxxxxx8E
Card Code Response: 	NO VALUE RETURNED
Reserved (40): 	NO VALUE RETURNED
Reserved (41): 	NO VALUE RETURNED
Reserved (42): 	NO VALUE RETURNED
Reserved (43): 	NO VALUE RETURNED
Reserved (44): 	NO VALUE RETURNED
Reserved (45): 	NO VALUE RETURNED
Reserved (46): 	NO VALUE RETURNED
Reserved (47): 	NO VALUE RETURNED
Reserved (48): 	NO VALUE RETURNED
Reserved (49): 	NO VALUE RETURNED
Reserved (50): 	NO VALUE RETURNED
Reserved (51): 	XXXX6679
Reserved (52): 	Visa
Reserved (53): 	NO VALUE RETURNED
Reserved (54): 	NO VALUE RETURNED
Reserved (55): 	NO VALUE RETURNED
Reserved (56): 	NO VALUE RETURNED
Reserved (57): 	NO VALUE RETURNED
Reserved (58): 	NO VALUE RETURNED
Reserved (59): 	NO VALUE RETURNED
Reserved (60): 	NO VALUE RETURNED
Reserved (61): 	NO VALUE RETURNED
Reserved (62): 	NO VALUE RETURNED
Reserved (63): 	NO VALUE RETURNED
Reserved (64): 	NO VALUE RETURNED
Reserved (65): 	NO VALUE RETURNED
Reserved (66): 	NO VALUE RETURNED
Reserved (67): 	NO VALUE RETURNED
Reserved (68): 	NO VALUE RETURNED
Merchant-defined (69): : 	FALSE
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I have tested the form at Authorize.net's Data Validation URL, but the entered values were returned in the test.
When I test locally, the form returns the values entered for expiration.

Here's my code for the month and year drop downs:

<select size="1" class=" validate['required']" title="" name="input_select_14">
    <option value=0 selected>Select month</option>
    <option value=01>01 January</option>
    <option value=02>02 February</option>
    <option value=03>03 March</option>
    <option value=04>04 April</option>
    <option value=05>05 May</option>
    <option value=06>06 June</option>
    <option value=07>07 July</option>
    <option value=08>08 August</option>
    <option value=09>09 September</option>
    <option value=10>10 October</option>
    <option value=11>11 November</option>
    <option value=12>12 December</option>
  </select>
  <div class="clear"></div><div id="error-message-input_select_14"></div></div><div class="ccms_form_element cfdiv_select" id="input_select_151_container_div" style=""><label>Expiration Year*</label><select size="1" class="validate['required']" title="" name="input_select_15">
<option value=0 selected>Select year</option>
<option value=2014>2014</option>
<option value=2015>2015</option>
<option value=2016>2016</option>
<option value=2017>2017</option>
<option value=2018>2018</option>
</select>
<div class="clear"></div><div id="error-message-input_select_15"></div></div>

Authorize.net assures me that nothing has changed on their end and that no one else has reported this issue.

Any thoughts on what's happening?

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Suddenly Auth.Net rejects my expiry info