fatal error: loading a php library in a plugin.. what am i missing here?

i've been coding a new plugin and testing what i'm coding on my test elgg site (1.8.15) and the plugin is using 2 php libraries which i have stored in the folder - /mod/myplugin/lib/

i added the lines:

    $base_dir = elgg_get_plugins_path() . 'myplugin/lib/';
    elgg_register_library('myplugin:image-api', $base_dir . "image_api.php");
    elgg_register_library('myplugin:account-api', $base_dir . "account_api.php");
    elgg_load_library('myplugin:account-api');
    elgg_load_library('myplugin:image-api');

to the start.php file for the plugin and for several days this was fine. then without any obvious cause, i started seeing fatal errors being generated in elgg which point to elgg being unable to load one of the library php files. i have looked on the server and the files are both present and both have the same permissions and the owner/permissions are the same as is being used by other elgg plugins that are functioning correctly. i have looked through the elgg code and php functions and logs and as yet have not found the cause for this..
nothing specifically changed on my server (that i am aware of) between the code functioning and the error being generated (though obviously something that i am unaware of has changed somehow).

fatal error.

InvalidParameterException:LibraryNotFound

InvalidParameterException Object
(
[message:protected] => InvalidParameterException:LibraryNotFound
[string:Exception:private] => exception 'InvalidParameterException' with message 'InvalidParameterException:LibraryNotFound' in /var/www/html/mysite/engine/lib/elgglib.php:121
Stack trace:
#0 /var/www/html/mysite/mod/myplugin/start.php(20): elgg_load_library('myplugin...')
#1 [internal function]: stw_init('init', 'system', NULL)
#2 /var/www/html/mysite/engine/lib/elgglib.php(784): call_user_func_array('stw_init', Array)
#3 /var/www/html/mysite/engine/start.php(116): elgg_trigger_event('init', 'system')
#4 /var/www/html/mysite/engine/handlers/page_handler.php(41): require_once('/var/www/html/m...')
#5 {main}
[code:protected] => 0
[file:protected] => /var/www/html/mysite/engine/lib/elgglib.php
[line:protected] => 121
[trace:Exception:private] => Array
(
[0] => Array
(
[file] => /var/www/html/mysite/mod/myplugin/start.php
[line] => 20
[function] => elgg_load_library
[args] => Array
(
[0] => myplugin:image-api
)

)

[1] => Array
(
[function] => myplugin_init
[args] => Array
(
[0] => init
[1] => system
[2] =>
)

)

[2] => Array
(
[file] => /var/www/html/mysite/engine/lib/elgglib.php
[line] => 784
[function] => call_user_func_array
[args] => Array
(
[0] => myplugin_init
[1] => Array
(
[0] => init
[1] => system
[2] =>
)

)

)

[3] => Array
(
[file] => /var/www/html/mysite/engine/start.php
[line] => 116
[function] => elgg_trigger_event
[args] => Array
(
[0] => init
[1] => system
)

)

[4] => Array
(
[file] => /var/www/html/mysite/engine/handlers/page_handler.php
[line] => 41
[args] => Array
(
[0] => /var/www/html/mysite/engine/start.php
)

[function] => require_once
)

)

[previous:Exception:private] =>
)

 

anyone got any ideas as to what i can explore to resolve this?
thanks