Exception on Core 8.7.10 / 8.8.0-rc1 + Commerce 2.15

Properly configured module does not work, the following exception is thrown:
TypeError: Argument 1 passed to Drupal\commerce_price\Calculator::add() must be of the type string, integer given, called in /var/www/html.original/modules/contrib/commerce_otpsp/src/PaymentRequestBuilder.php on line 227 in Drupal\commerce_price\Calculator::add() (line 30 of /var/www/html.original/modules/contrib/commerce/modules/price/src/Calculator.php) #0 /var/www/html.original/modules/contrib/commerce_otpsp/src/PaymentRequestBuilder.php(227): Drupal\commerce_price\Calculator::add(0, '0') #1 /var/www/html.original/modules/contrib/commerce_otpsp/src/PaymentRequestBuilder.php(144): Drupal\commerce_otpsp\PaymentRequestBuilder->applyItems() #2 /var/www/html.original/modules/contrib/commerce_otpsp/src/PluginForm/SimplePayRedirectOffSitePaymentForm.php(55): Drupal\commerce_otpsp\PaymentRequestBuilder->getPaymentRequest(Object(Drupal\commerce_payment\Entity\Payment)) #3 /var/www/html.original/modules/contrib/commerce/modules/payment/src/Plugin/Commerce/InlineForm/PaymentGatewayForm.php(108): Drupal\commerce_otpsp\PluginForm\SimplePayRedirectOffSitePaymentForm->buildConfigurationForm(Array, Object(Drupal\Core\Form\FormState)) #4 /var/www/html.original/modules/contrib/commerce/modules/payment/src/Plugin/Commerce/CheckoutPane/PaymentProcess.php(218): Drupal\commerce_payment\Plugin\Commerce\InlineForm\PaymentGatewayForm->buildInlineForm(Array, Object(Drupal\Core\Form\FormState)) #5 /var/www/html.original/modules/contrib/commerce/modules/checkout/src/Plugin/Commerce/CheckoutFlow/CheckoutFlowWithPanesBase.php(559): Drupal\commerce_payment\Plugin\Commerce\CheckoutPane\PaymentProcess->buildPaneForm(Array, Object(Drupal\Core\Form\FormState), Array) #6 [internal function]: Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\CheckoutFlowWithPanesBase->buildForm(Array, Object(Drupal\Core\Form\FormState), 'payment') #7 /var/www/html.original/core/lib/Drupal/Core/Form/FormBuilder.php(520): call_user_func_array(Array, Array) #8 /var/www/html.original/core/lib/Drupal/Core/Form/FormBuilder.php(277): Drupal\Core\Form\FormBuilder->retrieveForm('commerce_checko...', Object(Drupal\Core\Form\FormState)) #9 /var/www/html.original/core/lib/Drupal/Core/Form/FormBuilder.php(218): Drupal\Core\Form\FormBuilder->buildForm(Object(Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\MultistepDefault), Object(Drupal\Core\Form\FormState)) #10 /var/www/html.original/modules/contrib/commerce/modules/checkout/src/Controller/CheckoutController.php(94): Drupal\Core\Form\FormBuilder->getForm(Object(Drupal\commerce_checkout\Plugin\Commerce\CheckoutFlow\MultistepDefault), 'payment') #11 [internal function]: Drupal\commerce_checkout\Controller\CheckoutController->formPage(Object(Drupal\Core\Routing\RouteMatch)) #12 /var/www/html.original/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(123): call_user_func_array(Array, Array) #13 /var/www/html.original/core/lib/Drupal/Core/Render/Renderer.php(573): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() #14 /var/www/html.original/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(124): Drupal\Core\Render\Renderer->executeInRenderContext(Object(Drupal\Core\Render\RenderContext), Object(Closure)) #15 /var/www/html.original/core/lib/Drupal/Core/EventSubscriber/EarlyRenderingControllerWrapperSubscriber.php(97): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) #16 /var/www/html.original/vendor/symfony/http-kernel/HttpKernel.php(151): Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() #17 /var/www/html.original/vendor/symfony/http-kernel/HttpKernel.php(68): Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object(Symfony\Component\HttpFoundation\Request), 1) #18 /var/www/html.original/core/lib/Drupal/Core/StackMiddleware/Session.php(57): Symfony\Component\HttpKernel\HttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #19 /var/www/html.original/core/lib/Drupal/Core/StackMiddleware/KernelPreHandle.php(47): Drupal\Core\StackMiddleware\Session->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #20 /var/www/html.original/core/modules/page_cache/src/StackMiddleware/PageCache.php(106): Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #21 /var/www/html.original/core/modules/page_cache/src/StackMiddleware/PageCache.php(85): Drupal\page_cache\StackMiddleware\PageCache->pass(Object(Symfony\Component\HttpFoundation\Request), 1, true) #22 /var/www/html.original/core/lib/Drupal/Core/StackMiddleware/ReverseProxyMiddleware.php(47): Drupal\page_cache\StackMiddleware\PageCache->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #23 /var/www/html.original/core/lib/Drupal/Core/StackMiddleware/NegotiationMiddleware.php(52): Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #24 /var/www/html.original/vendor/stack/builder/src/Stack/StackedHttpKernel.php(23): Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #25 /var/www/html.original/core/lib/Drupal/Core/DrupalKernel.php(694): Stack\StackedHttpKernel->handle(Object(Symfony\Component\HttpFoundation\Request), 1, true) #26 /var/www/html.original/index.php(19): Drupal\Core\DrupalKernel->handle(Object(Symfony\Component\HttpFoundation\Request)) #27 {main}.
Needs further investigation.