Troubleshooting

Flickr API changes or bugs

It does NOT mean your site has to be SSL as well. Your HTTP site can have embedded Flickr photos, just as before. Just upgrade the Flickr module if you haven't done so already.
D7: Upgrade to the latest dev or stable version.
D6: Upgrade to the latest dev.
Details:
http://code.flickr.net/2014/04/30/flickr-api-going-ssl-only-on-june-27th...
#2261371: Flickr API Going SSL-Only - Likely Impact?

Known Flickr API bugs (both solved and open) with an impact on the module

#2286823: Zero results when using tags (Flickr API bug)

Generic problem solving

• Try clearing both your site and browser cache.
• Use this tool to find your Flickr user or group id.
• BEFORE posting a support request READ the documentation and it's child pages thoroughly.
• BEFORE posting a support request related to configuration, check the necessary settings in the demo at simplytest.me. Complete the installation process there (just hit Save and continue on each step). If at the end of the install process you get a white screen, just reload the page.

Specific problems

Not working with Colorbox after installing it
Clear your site cache.

Flickr error 100: Invalid API Key (Key has invalid format)
Fill in some valid Flickr credentials at admin/config/media/flickr.
Get an API Key at https://www.flickr.com/services/apps/create/apply.

Unexpected display problems occur.
It should be noted that the cascading stylesheets defined by modules are by default loaded before theme CSS (see Drupal API). Flickr module's style might be overwritten by the theme stylesheets. Copy and paste the CSS code in the flickr.css file to the bottom of your theme's custom CSS file or use this solution.

Javascript problems (mainly conflicts with slider modules)
Use the latest dev version. The committed patch provided in #2301681: flickr_colorbox.js breaks javascript solves most of these.

'Undefined index' error after upgrade from 7.x-1.0 to a later version.
Re-save your block settings or upgrade to anything after version 7.x-1.5. See #2089575: Undefined index error after upgrade.

Uninstalling Flickr Field - module is not selectable because Drupal says the fields implemented by it are being used.

• First delete all Flickr fields in use on your content types at admin/structure/types 'Manage fields'.
• Run cron at admin/reports/status/run-cron

Getting debug info

• Enable Debug Output in the field group ADVANCED on the Flickr configuration page at admin/config/media/flickr.
• Re-save the node or block you have trouble with.
• Look at the argument values that are passed to the Flickr API request.

The debug info can be used in the API Explorer that you find at the bottom of an API method of choice. For example the photos for a user album are returned by flickr.photos.search. The corresponding API Explorer allows you to insert the debug info and see what is returned by Flickr. After selecting ' Do not sign call?' click the 'Call method' button.
 
An album shows not the photos as before while nothing has been modified
Occasionally a bug depends on a (temporary) malfunction of the Flickr API. An album on your site was working fine before, you made no changes but now it does not show the photos you would expect. If you are pretty confident that's the case and the API Explorer returns wrong results as well (see above), add the issue to an existing topic in the Flickr API group or open a new one. An example. Just for the purpose of informing other users, open an issue on Drupal as well and link to the the topic on Flickr. An example: #2286823: Zero results when using tags (Flickr API bug).

A slideshow ignores number, media, tags, location, date and sort settings.
The slideshow is an embedded object and completely handled by Flickr. There is already a feature request at #2258005: Slideshow that takes Flickr tags.

Number of photos in a Flickr Field photoset seems to be not configurable
Solution at #2298973: Only 6 images per photoset (after default changed).

Unable to retrieve photos. Flickr forbidden error.
It might depend on your webhost allowing only outgoing requests over cURL. Solution at #2295241: Unable to contact flickr and retrieve the pictures (try to use cURL when drupal_http_request returns a 403, forbidden).

How to get more than 99 photos in an album
Although not recommended, a solution is provided at #2299833: More than 99 images per page?. A better option is a pager for which there is already a feature request, see #2096881: Use a pager on a embedded photoset in the body.

A horizontal scrollbar only on the front page
This can happen on some not updated responsive themes but only on Google Chrome. See http://stackoverflow.com/q/14186277. In any case it's a theme bug. See for example #1983392: On webkit a horizontal scrollbar appears when the window is narrowed.. It can be solved easily by upgrading your theme or adding the following to the style.css file of your responsive theme as a workaround:

/* ---------- Custom ----------- */
body {
  overflow-x: hidden;
}

Background info

Most of the recent development efforts went into the sub-modules Flickr Filter and Flickr Block. Flickr Field is actively maintained but might be more subject to flaws. While not being a sub-module, that applies also to the display of Flickr photos in user profiles.

Feel free to help by providing patches. We always commit with attribution (credits).