InvalidArgumentException stating "You must use a valid URI scheme".

Here's the backtrace, maybe can help?

#0 /home/[redacted]/[subdomain-redacted]/backoffice/web/modules/contrib/rest_password_reset/rest_password_reset.tokens.inc(56): Drupal\Core\Url::fromUri()
#1 [internal function]: rest_password_reset_tokens()
#2 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(416): call_user_func_array()
#3 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(395): Drupal\Core\Extension\ModuleHandler->Drupal\Core\Extension\{closure}()
#4 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/Extension/ModuleHandler.php(415): Drupal\Core\Extension\ModuleHandler->invokeAllWith()
#5 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/Utility/Token.php(364): Drupal\Core\Extension\ModuleHandler->invokeAll()
#6 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/Utility/Token.php(241): Drupal\Core\Utility\Token->generate()
#7 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/Utility/Token.php(191): Drupal\Core\Utility\Token->doReplace()
#8 /home/[redacted]/[subdomain-redacted]/backoffice/web/modules/contrib/rest_password_reset/src/Plugin/rest/resource/ResetLink.php(141): Drupal\Core\Utility\Token->replace()
#9 [internal function]: Drupal\rest_password_reset\Plugin\rest\resource\ResetLink->get()
#10 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/modules/rest/src/RequestHandler.php(219): call_user_func_array()
#11 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/modules/rest/src/RequestHandler.php(68): Drupal\rest\RequestHandler->delegateToRestResourcePlugin()
#12 [internal function]: Drupal\rest\RequestHandler->handle()
#13 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array()
#14 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/Render/Renderer.php(638): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#15 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(121): Drupal\Core\Render\Renderer->executeInRenderContext()
#16 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext()
#17 /home/[redacted]/[subdomain-redacted]/backoffice/vendor/symfony/http-kernel/HttpKernel.php(181): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
#18 /home/[redacted]/[subdomain-redacted]/backoffice/vendor/symfony/http-kernel/HttpKernel.php(76): Symfony\Component\HttpKernel\HttpKernel->handleRaw()
#19 /home/[redacted]/[subdomain-redacted]/backoffice/web/modules/contrib/simple_oauth/src/HttpMiddleware/BasicAuthSwap.php(54): Symfony\Component\HttpKernel\HttpKernel->handle()
#20 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/StackMiddleware/Session.php(53): Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->handle()
#21 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(48): Drupal\Core\StackMiddleware\Session->handle()
#22 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/StackMiddleware/ContentLength.php(28): Drupal\Core\StackMiddleware\KernelPreHandle->handle()
#23 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(191): Drupal\Core\StackMiddleware\ContentLength->handle()
#24 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(128): Drupal\page_cache\StackMiddleware\PageCache->fetch()
#25 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/modules/page_cache/src/StackMiddleware/PageCache.php(82): Drupal\page_cache\StackMiddleware\PageCache->lookup()
#26 /home/[redacted]/[subdomain-redacted]/backoffice/vendor/asm89/stack-cors/src/Cors.php(53): Drupal\page_cache\StackMiddleware\PageCache->handle()
#27 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(48): Asm89\Stack\Cors->handle()
#28 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(51): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle()
#29 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/StackMiddleware/AjaxPageState.php(36): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle()
#30 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/StackMiddleware/StackedHttpKernel.php(51): Drupal\Core\StackMiddleware\AjaxPageState->handle()
#31 /home/[redacted]/[subdomain-redacted]/backoffice/web/core/lib/Drupal/Core/DrupalKernel.php(741): Drupal\Core\StackMiddleware\StackedHttpKernel->handle()
#32 /home/[redacted]/[subdomain-redacted]/backoffice/web/index.php(19): Drupal\Core\DrupalKernel->handle()
#33 {main}

You should provide a frontend uri under: /admin/config/services/rest-password-reset

This is a required setting but it's left intensionally empty on install as it would be hard to guess what the value should be on decoupled websites.

I could provide a more meaningful error on this particular error condition though.

Ok, my bad. I've solved it.

Didn't realise there was a doc's page. After reading, all went smoothly.