The following steps could fix the problem. Please perform them once.
First, connect to your FTP server and access the folder where your shop is located. Then drag all files in the _Hotfixes folder onto the FTP server. Now select the file Web.config, drag it to your computer and open it with an editor. Search for the word compilation then you should hit this line: <compilation targetFramework="4.6.1" numRecompilesBeforeAppRestart="253" batch="true" optimizeCompilations="true">.
Change the "253" to a "254" then upload the Web.config to the FTP server and reload the page of your shop, as soon as the page is loaded you drag the Web.config back to the PC. Next, change the "254" back to a "253", drag the Web.config to the FTP server and update the page.